To apply the Osteoarthritis Research Society International (OARSI) assessment system to an osteoarthritis model, 44 Wistar rats were randomized into treadmill-running exercise or control group. At 6, 8, and 10 weeks, medial knee joints were histopathologically evaluated, and aggrecan neoepitope and TUNEL staining were performed. Cartilage changes in exercise group were histopathologically and histochemically compatible with early OA. Total modified Mankin system (MMS) scores were significantly higher at all time points (each P ≤ 0.01) in exercise than in control group. However, only tibial OARSI scores of runners were higher at 10 weeks (P < 0.05), although OARSI scores were found to be significantly correlated with MMS scores. Both total MMS (Spearman's coefficient ρ = 0.786) and OARSI scores (ρ = 0.443 for femoral; ρ = 0.604 for tibial) were significantly associated with the exercise duration. In conclusion, the OARSI system may not be sensitive to early OA changes induced by treadmill exercise.