Objectives: the purpose of the study was to evaluate the effect of locally delivered bisphosphonate alendronate on osteoclast numbers and bone formation following bone grafting in the rat mandible. Methods: twenty-six rats were divided into two groups: 13 experimental and 13 control rats. The rats in the experimental group were grafted with bisphosphonate alendronate and BioGran® in the lesion. The rats in the control group were grafted with BioGran® and saline. Results: the results showed that the mean (±SD) of the osteoclast numbers in the experimental group was 0.37±0.11 cells/mm2 which was not significantly different from the control group (i.e. 0.37±0.14 cells/mm2). However, the mean (±SD) of the amount of new bone formation in the experimental and control groups were 6.12±2.14 and 4.04±1.39 mm2, respectively, and the difference was statistically significant (P< 0.05). Conclusions: we concluded that locally delivered bisphosphonate alendronate as adjunct to BioGran® could induce more bone formation compared with BioGran® and saline, and suggested the possibility of local application of bisphosphonate alendronate as adjunct to guided bone regeneration procedures. This study was supported by The Khon Kaen University's Graduate Research Fund, Grant 4612101.