The objective of this study was to evaluate the biocompatibility and effects of the particulated and surface modified Ostrich eggshell (OES) as bone graft substitutes in healing of calvarial defects in rats. Additionally we compared the bone forming ability of the surface modified OES to that of BioCoral (Inoteb, France), which has the same chemical compositions as OES, calcium carbonate (CaCO3). Surface modified OES particles were fabricated by alkaline etching (microroughened-OES) and biomimetic calcium phosphate coating (CaP coated-OES). 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay was performed to evaluate cell viability at 1, 4, and 7days using ROS 17/2.8 cells, a rat osteosarcoma cell line. To evaluate the bone forming ability, surface modified OES and BioCoral were filled in the calvarial defects of fourteen adult male Sprague-Dawley rats. After 4 weeks of healing, animals were sacrificed and evaluated histologically and histomorphometrically. The MTT assay indicated the increases in viable cell numbers of all groups according to the time, and significantly increased cell numbers were observed on CaP coated-OES and BioCoral at 7 day (P<0.01). The histomorphometric results showed that significantly increased amount of new bone formation in the microroughened-OES, CaP coated-OES, and BioCoral grafted defects compared with unfilled defects (P<0.0001). The degree of ossification was most prominent in CaP coated-OES grafted defects. The results of this study suggest that surface modified OES particles are biocompatible and yield favorable bone formation in rat calvarial defects. In conclusion, surface modified OES grafts may be considered as effective osteoconductive grafting materials, but further studies are needed to produce more optimal surface modification and confirm long-term results.