Objective: was directed to evaluate platelet rich fibrin alone and in combination with xenograft as an alternative to bone substitute in treatment of critical sized bone defects in dogs. Material and methods:8 dogs were selected for this study in which diabetes mellitus was induced. Root canal treatment was done in the 2nd and 4th premolar teeth of both sides then 4 critical sized bone defects were created in relation to the distal root of the selected teeth. According to the bone cavity filling materials there was 4 groups xenograft, xenograft and PRF, PRF and blood clot groups. Dogs were sacrificed at 4 days,2,5and 9 weeks the mandibles were harvested for DEXA evaluation. Results: bone density measurement at 9 weeks was significant with Xenograft and PRF group than Xenograft, PRF and blood clot groups, and at the same time there was no significance between Xenograft and PRF groups. Conclusion: PRF is a valuable material that can be used as a sole grafting material in periradicular surgery to improve bone regeneration.