Purpose: Synthetic bone products such as biphasic calcium phosphate (BCP) are mixtures of hydroxyapatite (HA) and ${eta}$-tricalcium phosphate (${eta}$- TCP). In periodontal therapies and implant treatments, BCP provides to be a good bone reconstructive material since it has a similar chemical composition to biological bone apatites. The purpose of this study was to compare bone regeneration capacity of two commercially available BCP. Methods: Calvarial defects were prepared in sixteen 9-20 months old New Zealand White male rabbits. BCP with HA and ${eta}$- TCP (70:30) and BCP with Silicon-substituted hydroxyapatite (Si-HA) and ${eta}$-TCP (60:40) particles were filled in each defect. Control defects were filled with only blood clots. Animals were sacrificed at 4 and 8 week postoperatively. Histomorphometric analysis was performed. Results: BCP with HAand ${eta}$- TCP 8 weeks group and BCP with Si-HA and ${eta}$- TCP 4 and 8 weeks groups showed statistically significant in crease (P <0.05) in augmented area than control group. Newly formed bone area after 4 and 8 weeks was similar among all the groups. Residual materials were slightly more evident in BCP with HA and ${eta}$- TCP 8 weeks group. Conclusions: Based on histological results, BCP with HA and ${eta}$- TCP and BCP with Si-HA and ${eta}$- TCP appears to demonstrate acceptable space maintaining capacity and elicit significant new bone formation when compared to natural bone healing in 4 and 8 week periods.