Objective To investigate the characteristics and risk factors for allogeneic transfusion after unilateral total knee arthroplasty (TKA). Methods 852 patients (663 female and 189 male) underwent primary unilateral TKA from January 2014 to December 2014 were included. Average age of included patients were 64.9±7.9 years old (22-87). 829 patients were osteoarthritis, others rheumatoid arthritis. The ASA score, BMI, doctor groups, diabetes, hypertension, thrombus (duplex color Doppler ultrasonography), pre-HGB, pre-HCT, pre-TP, pre-Cr, pre-BUN, pre-PT, operation time, starting MABP of the operation, anesthesia and TXA were collected. Potential risk factors for allogeneic transfusion were analyzed statistically via univariate and multivariate regression analysis. Results The preoperative hemoglobin level in 71 (8.3%) patients were lower than that in normal (male < 120 g/L, female <110 g/L). The hematokrit in 27 (3.2%) patients were lower than that in normal (male <40%, female <37%). TXA was used in 740 (86.9%) patients during the operation. Allogeneic transfusion was performed in 202 (23.7%) the patients after TKA. The differences in the following items within two groups were statistically significant via univariate analysis (P<0.05), female and male, ≥70 and <70 years, pre-HGB normal and low, pre-HCT normal and low and with and without TXA. Female [OR= 2.283, 95%CI (1.405, 3.711)], patient age of 70 years or older [OR=2.048, 95%CI (2.064, 4.292)], preoperative hemoglobin level low [male <120 g/L, female <110 g/L, OR=1.506, 95%CI (1.376, 4.427)] and preoperative hematokrit below normal [male <40%, female <37%, OR=3.412, 95%CI (1.086, 6.591)] were independent predictors for postoperative allogeneic transfusion in multivariate regression analysis. Conclusion The allogeneic transfusion rate after unilateral TKA was 23.7%. Female, older than 70 years and preoperative anemia were independent predictors for postoperative allogeneic transfusion after TKA. TXA can effectively decrease the postoperative allogeneic transfusion rate and the amount of transfusion. Key words: Arthroplasty, replacement, knee; Blood transfusion; Risk factors