Purpose: To estimate the efficacy of various bone grafting materials in the healing of 8-mm-diameter critical-size defects (CSD) in guinea pig calvaria. Methods: In this randomized trial study, critical-size defects were created in the calvaria of 36 guinea pigs 6 months of age. Animals were assigned into three groups and each received one of three experimental protocols. In protocol A, the right-side defect was filled with macroporous biphasic calcium phosphate (MBCP Gel) and the left side was left empty as a control. In protocol B, the right-side defect was filled with demineralized freeze-dried bone allograft (DFDBA) and the left side was filled with Stypro. In protocol C, the right-side defect was filled with Bio-Oss and the left side was filled with autogenous bone. The percentage of new bone formation was evaluated histomorphometrically after 8 weeks. Results: The mean bone formation was 68.19% for autogenous bone, 66.96% for MBCP Gel, 57.28% for Bio-Oss, 50.19% for DFDBA, 18.79% for Stypro, and 10.61% for the empty control. Except for the MBCP Gel and autogenous bone groups (P = .6), the differences between the other groups were statistically significant (P .05). Conclusion: The results of this study indicated that all the testing materials had different capacities to produce new bone in CSD of guinea pig calvaria. MBCP Gel showed promising results in producing new bone proportionate to the autogenous bone graft group.