Objectives The effect of amount of correction on postoperative changes in the posterior tibial slope (PTS), patellar height (PH) and clinical outcomes following biplanar open-wedge high tibial osteotomy (OWHTO) were evaluated. Method This study included 64 knees (30 left, 34 right) of 64 patients (mean age 48.5±6.8 years, 20 males, 44 females) with varus malalignment and symptomatic isolated medial joint osteoarthritis who underwent OWHTO. According to the amount of correction angles, all patients were divided into three groups: large correction angle (LCA) group (>14°), medium correction angle (MCA) group（10°～14°）and small correction angle (SCA) group (<10°). All patients were clinically assessed according to the Lysholm score, hospital for special surgery knee score (HSS), and knee society score (KSS) prior to and after surgery. For radiographic analysis, we measured the PTS，PH [Insall-Salvati index (ISI), Blackbume-Peel index (BPI)]. The pre-post difference of PTS, ISI and BPI was calculated by subtracting the post-OWHTO value to the pre-OWHTO value in three groups respectively. The preoperative, postoperative, and difference of PTS, ISI and BPI values were analyzed according to the correction angle. The mean follow-up period was 27.7 months(SD, 3.9; range 12-49 months). Results Radiologically, PTS increased and PH decreased after surgery on the whole ( p ＜0.05). The relationship between amount of correction and slope increasing is significant ( p ＜0.001). Furthermore, the pairwise difference between the LCA group and SCA group, MCA group are significant respectively ( p ＜0.05). In terms of PH, the LCA group yielded ISI and BPI that were significantly different from baseline for SCA group and MCA group. In addition, the pairwise difference between the SCA group and LCA group in ISI and BPI are significant ( p =0.031). Clinically, significant improvements were observed in postoperative clinical scores of the Lysholm score, HSS, and KSS ( p <0.05). Fifty-nine patients (92.2%) reported satisfaction with surgery. However, no correlation was found between changes in PTS and PH with postoperative knee score. No severe adverse complications were observed. Conclusions The amount of correction angle is a significant factor affecting the PTS and PH in OWHTO. With increased correction angle, the likelihood of increasing the PTS and decreasing the PH increases. Special attention should be paid to keep PTS and PH unchanged in cases where large corrections are required. Otherwise, closing wedge osteotomy or other intraoperative effective measures are supposed to be adopted.