Objective:To observe the expression of C telopeptide of type Ⅱcollagen(CTX-Ⅱ)in serum and protein extract from joints of experimental osteoarthritis(OA),and to explore the value of CTX-Ⅱ as a biomarker in early diagnosis and condition assessment. Methods:96 adult SD rats were randomly divided into two groups:Sham-operated group (n=48),OA model group(n=48). OA was induced by unilateral surgical transection of the anterior cruciate ligament. Joints were isolated 0,3days,1,2,4,8weeks after surgery for protein extraction and histology. CTX-Ⅱ in protein extracts and serum was measured by ELISA. Results:Levels of CTX-Ⅱ in serum and protein extract were both significantly increased in OA model group compared with sham-operated group on each time point. The pathological features of OA were observed during 2~8 weeks and became gradually more serious. The changes in level of CTX-Ⅱ in serum and protein extraction showed a close correlation. Conclusion:Serum CTX-Ⅱ level increases before the appearance of characteristic pathology of OA and is in coincidence with the changes of CTX-Ⅱ in the tissue. It can be a sensitive reference indicator for early diagnosis and condition evaluation of OA.