Objective: To evaluate the prognostic value of CA9 in predicting postoperative recurrence， metastasis or survival in patients with clear cell renal cell carcinoma(ccRCC). Methods: The clinical pathological data from 231 patients with ccRCC who underwent radical or partial nephrectomy from January 1, 2017 to December 31, 2020 in Ruijin Hospital,Shanghai Jiao Tong University were retrospectively analyzed. The patients were followed up for the overall survival (OS),disease-free survival (DFS),and tumor recurrence or metastasis. All pathological sections and wax blocks of tumor issue were collected to make tissue microarray, and immunohistochemical analysis of tissue microarray was performed using CA9 polyclonal antibody. SPSS statistical software multivariate Cox and Kaplan-Meier were used to evaluate the correlation of CA9 expression with clinicopathological features, OS and DFS. Results: Of the 231 ccRCC patients had a median follow-up of 37 months (13-54 months). It revealed that postoperative recurrence and/or metastasis occurred in 38 cases, inclu-ding 6 cases with both recurrence and metastasis, 3 cases with only recurrence, 29 cases with only metastasis, and 8 deaths. Among 231 patients, CA9 positive expression rate was 94%(217/231), of which 153 (66%) were with cells stained with CA9 ≥85% and 78 (34%) were stained with CA9 <85%. Chi-square analysis showed that the expression of CA9 was not only correlated with T stage, ISUP nuclear grade, sarcomatoid or rhabdomyoid differentiation, necrosis, capsule invasion, vascular infiltration, but also correlated with tumor recurrence, metastasis and survival status. There was no relationship between CA9 expression and age, sex and tumor location (P>0.05). Univariate Kaplan-Meier analysis and multiva-riate Cox analysis showed that low expression of CA9 (<85%) was a poor prognostic factor for postoperative overall survival (OS) and disease-free survival (DFS) in patients with ccRCC, with hazard ratios of 6.211 (P=0.002) and 8.980 (P<0.001), respectively. Conclusions: In ccRCC, low expression of CA9 is associated with poor OS and DFS, and is an independent risk factor for OS and DFS in postoperative patients with ccRCC, which may serve as a useful prognostic indicator.