Objective: The aim of this observational study was to investigate the risk factors of postoperative valgus malalignment after mobile-bearing medial unicompartmental knee arthroplasty (UKA). Methods: We retrospectively evaluated radiographic and surgical characteristics in 122 consecutive Oxford phase 3 UKAs. According to postoperative hip-knee-ankle angle (HKAA), 24 knees were sorted into group valgus with HKAA > 180 degrees and 98 knees were sorted into group non-valgus with HKAA <= 180 degrees. Logistic regression was performed to analyze risk factors including age, gender, BMI, side, preoperative limb alignment HKAA, preoperative LDFA, MPTA, FTFA, thickness of polyethylene bearing insert, tibial prothesis size, femoral prothesis size, medial tibial cut thickness, thickness of distal femoral mill, prothesis angle of coronal, and sagittal plane. Results: The mean mechanical preoperative HKAA of 174.39 degrees +/- 4.23 degrees was corrected to 178.18 degrees +/- 3.49 degrees postoperatively (t=- 13.45, p=0.000). The mean of postoperative HKAA in valgus group and non-valgus group was 183.45 +/- 2.21 degrees and 176.88 +/- 2.35 degrees, respectively (t=12.44, p=0.000). After statistical analysis with univariate analysis, eight risk factor variables among 16 independent variables were identified as potential predictors with p value0.1. Multivariate logistic regression analysis for these eight potential predictors revealed that tibial cut (p=0.046), LDFA (p=0.003), MPTA (p=0.011), and FTFA (p=0.008) were significant risk factors predicting postoperative valgus malalignment after mobile-bearing UKA. Conclusions: Preoperative smaller LDFA, FTFA, larger MPTA and less medial tibial cut thickness were significantly associated with postoperative valgus malalignment in mobile-bearing UKA.