The purpose of this study was to assess the fracture resistance of roots that were prosthetically restored with intra-radicular posts of different lengths. Forty-five bovine incisors were sectioned 17 mm from their apices, endodontically treated and randomly divided into three experimental groups: GI, fibreglass posts luted at a depth of 12 mm; GII, 8 mm and GIII, 4 mm. All posts were luted with dual resin cement. Resin composite cores were prepared with standardized measurements, and all teeth were restored with metal crowns. The samples were submitted to the fracture resistance test in a universal testing machine, at an angle of 135 degrees and speed of 0.5 mm min(-1), until fracture occurred. The data, in MPa, were submitted to the analysis of variance (ANOVA) followed by Tukey's test (alpha = 0.01). No statistically significant difference (P > 0.01) was found between GI (129.72 +/- 12.14) and GII (154.3 +/- 12.08), which presented the highest fracture resistance values. Group GIII (100.5 +/- 8.07) showed lower fracture resistance (P < 0.01) when compared with GI and GII. Within the limitations of this study, it was concluded that the post lengths influenced the fracture resistance of prosthetically restored roots. These results suggest that it is not necessary to perform excessive intra-radicular post-space preparation to improve the fracture resistance of roots.372116122