The purpose of this radiologic, longitudinal study was to estimate the effects of the platelet rich plasma (PRP) on improvement of the defective articular cartilage in an osteoarthritis (OA) model. The PRP is an autologous product that concentrates a high number of platelets in a small volume of plasma. It mimics the last step of the coagulation cascade, leading to formation of a fibrin clot, which consolidates and adheres to the application site in a short period of time. Twenty adult guinea pigs (treatment group, n = 10; control group, n = 10) were anesthetized and the anterior cruciate ligament of the left knee was transected through a para-patellar approach. The animals were allowed to resume normal cage activity for 12 weeks. For the preparation of PRP, 2 ml blood was collected into a Na-citrate tube by direct heart puncture. Twelve weeks post-operation PRP was injected into the OA joint of the treatment group only. The animals were observed for further 8 weeks. At 20 weeks radiological score of OA increased in control group. Reduction in the degree of osteophyte formation, and subchondral bone sclerosis were detected in the PRP-treated joints. In conclusion, this study shows beneficial effects of the PRP on improvement of the defective articular cartilage in the OA joint.