The ultrasonic methods are widely used for evaluation of materials for its inexpensive and practical features. Among the largely used ultrasonic methods is the Critically Refracted Longitudinal Waves (LCR), which applies longitudinal bulk waves travelling right below the surface, parallel to it. The method can be used to measure stresses near the surface through acoustoelasticity. However, the materials microstructure directly influences the propagation of these waves. The mean grain size is one of the factors that affect the wave speed. The purpose of this paper is to evaluate the effect of mean austenitic grain size in the time of flight (TOF) of LCR wave. Samples (CDPs) of ASTM A36 steel were manufactured with dimensions of 5/8' X 6' X 12'. These were submitted to heat treating at the different temperatures ranges and cooled in air to generate different grain sizes. The results were compared to the grain size estimated in the literature and obtained from optical microscopy. The study showed a clear dependence of TOF on mean austenitic grain size for ASTM A36 steel. Such effect has to be studied to allow stress measurements with Lcr waves. Copyright © 2012 by ASME.10 6369Bray, D.E., Stanley, R.K., Nondestructive evaluation: A tool in design (1997) Manufacturing, and Service, p. 586. , Revised Edition. Ed. CRC Press, Boca RatonBray, D.E., Tang, W., Subsurface Stress Evaluation in Steel Plates and Bars using the Lcr Ultrasonic Wave (2001) Nuclear Engineering and Design, 207, pp. 231-240Artimowicz, D., Hutchinson, B., Nogues, M., Effects of crystallographic texture on ultrasonic wave velocities in steels (2002) Materials Science and Technology, 18, pp. 1142-1146Santos, R.A., (2007) Effect of Temperature on the Stress Measurement by LCR Waves in API 5L X70 Steel, p. 127. , (In Portuguese), M. Sc. Thesis, State University of Campinas, Campinas, S. P., BrazilFraga, R.S., (2007) Evaluation of Penetration Depth of Critically Refracted Longitudinal Waves, p. 142. , (In Portuguese), M. Sc. Thesis, State University of Campinas, Campinas, S. P., BrazilPalanichamy, P., Joseph, A., Jayakumar, T., Raj, B., Ultrasonic velocity measurements for estimation of grain size in austenitic stainless steel (1995) NDT & e International, 28 (3), pp. 179-185. , Great BritainAhn, B., Lee, S.S., Hong, S.T., Kim, H.C., Kang, L.S.-J., Application of the acoustic resonance method to evaluate the grain size of low carbon steels (1999) NDT & e International, 32, pp. 85-89Ahn, B., Lee, S.S., Effect of microstructure of low carbon steels on ultrasonic attenuation (2000) IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 47 (3), pp. 620-629Bouda, A.B., Lebaili, S., Benchaala, A., Grain size influence on ultrasonic velocities and attenuation (2003) NDT & e International, 36, pp. 1-5Sarpün, I.H., Kiliçkaya, M.S., Tuncel, S., Mean grain size determination in marbles by ultrasonic velocity techniques (2005) NDT & e International, 38 (1), pp. 21-25. , Great BritainÜnal, R., Sarpün, I.H., Yalim, H.A., Erol, A., Özdemir, T., Tuncel, S., The Mean grain size determination of boron carbide (b4c)-aluminium (al) and boron carbide (b4c)-nickel (ni) composites by ultrasonic velocity technique (2006) Materials Characterization, 56 (3), pp. 241-244. , USASarpün, I.H., Özkan, V., Tuncel, S., Ünal, R., Determination of mean grain size by ultrasonic methods of tungsten carbide metal matrix composites sintered at various temperatures (2007) 4th International Conference on NDT, pp. 1-5. , Chania, Crete-GreeceÖzkan, V., Sarpün, I.H., Examining with the sintered temperature of mean grain size of b4c-al-ni composites by ultrasonic techniques (2012) Proceedings of the International Congress on Advances in Applied Physics and Materials Science, 121, pp. 184-186. , Antalya, TurkeyAndrino, M.H., (2007) Application of Critically Refracted Longitudinal Waves for the Stress Measurement in Pipelines, p. 203. , (In Portuguese), Ph. D. Thesis, State University of Campinas, Campinas, S. P., BrazilSilva, C.A.L.V., Mei, P.R., (2006) Tratamentos Térmicos, pp. 83-146. , In: Aços e Ligas Especiais., 2nd. ed., Sao Paulo: Ed. Edgard Blücher, cap. 3Cunha, A.A.P., (2009) Effects of Molybdenum and Niobium Addition on the Microstructure and Mechanical Properties of Hot Rolled 0,5 %C Steel, p. 88. , (In Portuguese), M. Sc. Thesis, State University of Campinas, Campinas, S. P., BrazilFraga, R.S., Santos, A.A., Andrino, M.H., Temperature effect on the measurement of stresses in pipelines using ultrasonic lcr waves (2008) IMECE International Mechanical Engineering Congress and Exposition, pp. 361-367. , Boston, Massachusetts, USABuenos, A.A., (2010) Evaluation of Residual Stress Generated by Machining Process Using Acustoelasticity, p. 179. , (In Portuguese), M. Sc. Thesis, State University of Campinas, Campinas, S. P., Brazil