Purpose: To evaluate the effect of the resin cement thickness (RCT) on bond strength (BS) and gap formation (GF) of fiber posts bonded to root dentin. Materials and Methods: The roots of 24 extracted human mandibular premolars were treated endodontically and the post spaces were prepared using drills with different diameters according to the following groups (n = 8): well adapted (WA), moderately well adapted (MA) and poorly adapted (PA). The fiber glass posts were cemented (Excite DSC and Variolink II) in accordance with the manufacturer's instructions. After 1 week, the roots were transversely sectioned into six 1-mm-thick disks and photographed using an optical microscope to determine the RCT. After this, epoxy resin replicas of each sample were observed using SEM, and the mean length and width of the gaps formed in the cement/dentin/post interface were measured. Finally, each sample was subjected to the push-out test (0.5 mm/min), and the data were statistically analyzed using one-way ANOVA and Tukey's tests at the 5% level of significance. Results: The lowest RCT (μm) was observed for WA and the highest for PA. The group MA showed an intermediate value. Significantly higher push-out BS values were observed for WA compared to the other groups. The MA and PA groups were statistically similar. The highest mean gap length (%) and width (μm) were observed for PA. The groups MA and WA were statistically similar. Conclusion: Lower resin cement thickness resulted in better fiber post adhesion, that is, in higher bond strength and less gap formation. Keywords: bond strength, fiber posts, gap, resin cements, root dentin, scanning electron microscopy