The major goals of periodontal therapy is the functional regeneration of periodontal supporting structures already destructed by periodontal disease as well as the reduction of signs and symptoms of progressive periodontal disease. There have been many efforts to develop materials and therapeutic methods to promote periodontal wound healing. There have been increasing interest on the chitosan made by chitin. Chitin is second only to cellulose as the most abundant natural biopolymer. It is a structural component of the exoskeleton of invertebrates(e.g., shrimp, crabs, lobsters), of the cell wall of fungi, and of the cuticle of insects. Chitosan is a derivative of chitin made by deacetylation of side chains. Many experiments using chitosan in various animal models have proven its beneficial effects. The aim of this study is to evaluate the osteogenesis of chitosan on the calvarial critical size defect in Sprague Dawley rats. An 8 mm surgical defect was produced with a trephine bur in the area of the midsagittal suture. The rats were divided into two groups: Untreated control group versus experimental group with 50mg of soluble chitosan gel. The animals were sacrificed at 2, 4 and 8 weeks after surgical procedure. The specimens were examined by histologic, histomorphometric and radiodensitometric analyses. The results are as follows: 1. The length of newly formed bone in the defects was $102.91{pm}25.46{mu}m$, $219.46{pm}97.81{mu}m$ at the 2 weeks, $130.95{pm}39.24{mu}m$, $212.39{pm}89.22{mu}m$ at the 4 weeks, $181.53{pm}76.35{mu}m$ and $257.12{pm}51.22{mu}m$ at the 8 weeks in the control group and experimental group respectively. At all periods, the means of experimental group was greater than those of control group. But, there was no statistically significant difference between the two groups. 2. The area of newly formed bone in the defects was $2962.06{pm}1284.48{mu}m^2$, $5194.88{pm}1247.88{mu}m^2$ at the 2 weeks, $5103.25{pm}1375.88{mu}m^2$, $7751.43{pm}2228.20{mu}m^2$ at the 4 weeks and $8046.20{pm}818.99{mu}m^2$, $15578.57{pm}5606.55{mu}m^2$ at the 8 weeks in the control group and experimental group respectively. At all periods, the means of experimental group was greater than those of control group. The experimental group showed statistically significant difference to the control group at the 2 and 8 weeks. 3. The density of newly formed bone in the defects was $14.26{pm}6.33%$, $27.91{pm}6.65%$ at the 2 weeks, $20.06{pm}9.07%$, $27.86{pm}8.20%$ at the 4 weeks and $22.99{pm}3.76%$, $32.17{pm}6.38%$ at the 8 weeks in the control group and experimental group respectively. At all periods, the means of experimental group was greater than those of control group. The experimental group showed statistically significant difference to the control group at the 2 and 8 weeks. These results suggest that the use of chitosan on the calvarial defects in rats has significant effect on the regeneration of bone tissue in itself