In this work the effect of 2-phenyl-l-hydrazine carboxamide on the corrosion of mild steel in solutions of phosphoric acid (H3PO4 has been investigated in relation to the concentration of the inhibitor and acid, by weight loss, potentiodynamic polarization, and AC impedance (EIS) measurements. Results obtained revealed that this compound is a good mixed -type inhibitor. Increasing the acid concentration increased the metal corrosion but did not affect on the inhibitor efficiency. The optimum inhibitor concentration remained 150 ppm for various concentration of H3PO4. The surface morphologies of specimens after 1 hour of immersion in various solutions were studied by scanning electron microscopy (SEM).The Langmuir adsorption isotherm fitted well with the experimental data.