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BACKGROUND: The biological functions of platelet-rich plasma (PRP) are affected by multiple factors, such as individual difference, PRP concentration, PRP carder, PRP-activated methods and so on. OBJECTIVE: To evaluate the effect of PRP, activated by different concentrations of thrombin, on the repair of cranial defects. METHODS: Whole blood of the central artery of rabbit ears was extracted to prepare PRP, which was then diluted so that the final platetet count was about 5 times of the whole blood. Four whole-thickness layer of cranial defects at an 8-mm diameter were created in 16 New Zealand rabbits and randomly grafted with β-tdcalcium phosphate (β-TCP) and PRP, activated by 60 U/mL. thrombin; β-TCP and PRP, activated by 1 000 U/mL thrombin; β-TCP and PRP; β-TCP alone. At 1 and 3 months following implantation, X-ray analysis and microscopic observation were performed to onserve cranial repair, the area percent of new bone formation was calculated. RESULTS AND CONCLUSION: At one month post-surgery, the edge of defects was clear in each group, with varying degrees of new bone formation surrounding the defects, β-TCP particles partially degraded and the degradation lesion was replaced by new bone, only a small amount of bone lacunae was seen, fiber wrapped around the defect center β-TCP, only a small number of specimens showed new bone formation; X-ray showed a clear boundary and uniform defect density; the percentage of new bone formation in the PRP groups were higher than β-TCP groups (P < 0.05). However, there was no significant difference between PRP group groups (P > 0.05). At 3 months post-surgery, the defect boundary was unclear in each group, the new bone formation increased, the β-TCP particles surrounding defects partially or all degraded and were replaced by new bones, some regions appeared trabecular bone, bone lacuna in new bone was increased, the central defect of the majority of specimens exhibited new bone formation; X-ray showed defect boundary was unclear in each group, defect surrounding density was higher than the center defect, and bone mineral density was equivalent to other normal parts; the percentage of new bone formation in the PRP groups was significantly higher than that in the β-TCP groups (P < 0.05), PRP +β-TCP group was higher than the other 3 groups (P <0.05), there was no significant difference between two thrombin groups (P > 0.05). It is indicated that although PRP improves the repair of cranial defects, 60 and 1 000 U/mL of thrombin has no effects on PRP rapairing cranial defects in New Zealand white rabbits, compared with PRP+β-TCP group, possible the absence of the optimal concentration of thrombin.