Although it is possible to obtain a reliable bond between enamel and composite resin, the ideal bonding method of composite resin to dentin still needs to be developed. Variables such as the type of adhesive system used and type of dentin substrate can influence on the adhesion. Objective: The purpose of this study was to compare the shear bond strength of composite resin restorations to human and bovine dentin using three adhesive systems. Material and methods: Fifteen human third molars sectioned into two halves and 30 bovine incisors were cut into blocks (4x4mm), embedded in acrylic resin and ground flat to expose the dentin. The specimens were randomly divided into six groups (n = 10): group 1 – human dentin using Scotchbond Multi-Purpose; group 2 – human dentin using Adper Single Bond 2; group 3 – human dentin using Adper Prompt L-Pop; group 4 – bovine dentin using Scotchbond Multi-Purpose; group 5 – bovine dentin using Adper Single Bond 2; group 6 – bovine dentin using Adper Prompt L-Pop. After composite resin restoration procedure, the specimens were stored into distilled water for 24h at 37ºC and then submitted to the shear test using a universal testing machine. The failure patterns were examined microscopically and classified as adhesive, cohesive in resin, cohesive in dentin or both, and mixed. The ANOVA (two-way) and Tukey’s post hoc were used. Chi-square test for independence was used for analysis of failure mode. The significance level was set at 5%. Results: A significant difference in shear bond strength was observed among adhesive systems (p = 0.031), with higher values for one-bottle adhesive (8.87±2.72) and lower for self-etching (6.38±3.15), and between the two types of substrate (p = 0.018), with higher values for human dentin. However, there was no significant difference for the adhesive system/substrate interaction (p = 0.11). Adhesive failure was the predominant failure mode for all adhesive systems and for the two substrates. Conclusion: Shear bond strength was different between human and bovine substrates and for the adhesive system used.