Objective: This study evaluated the effects of different post diameter and oversized post spaces on the push-out bond strength of a fiber post to dentin. Method and Materials: Fifty extracted human maxillary central incisors and canines were divided into five groups and submitted to the push-out test (0.5 mm min-1). Groups 1, 2, and 3 were restored using a fiber post size that was identical to the drill size (sizes 1, 2, and 3, respectively), and groups 4 and 5 were both prepared with drill size 3, and restored using the size 2 and 1 fiber post, respectively. The fiber posts were cemented using self-adhesive dual-polymerized resin cement (RelyX Unicem). The data were analyzed using a one-way analysis of variance (ANOVA) and the Tukey test. Results: There were no significant differences in the mean values for push-out bond strength between groups with different post diameters (P > .05). However, the push-out bond strengths were significantly different between groups with different cement thicknesses, and group 4 yielded the highest bond strength (11.7 ± 0.4 MPa). For all groups, the apical third had the lowest bond strength value (P .05). Conclusion: The highest push-out bond strength values were obtained when one incremental oversized post space was used. Clinically, fiber post space has to provide an optimum cement thickness (around 120 µm) for adequate cementation. Palabras clave: bond strength, cement thickness, fiber post, push-out, self-adhesive cement