This study evaluated the functional outcomes of proximal humeral fractures treated with locking plates and determined the prognostic factors for successful clinical outcomes. We included 44 patients with 45 unstable proximal humeral fractures. There were 19 two-part, 22 three-part, and 4 four-part fractures. Outcomes were analyzed by use of multivariate linear regression with several different combinations of 7 independent variables: age, fracture pattern, osteoporosis, calcium sulfate graft, rotator cuff suture loop, head-neck shaft angle, and comorbidity. The mean head-shaft angle was 132.4° after the operation and decreased to 127.7° at final follow-up. Five patients showed considerable loss of fixation. The analysis showed that delay in rehabilitation because of comorbidities and decreased head-neck shaft angle induced by lack of medial support were the primary prognostic factors that led to poor outcomes. The absence of comorbidity and the restoration of the medial metaphysis were the most reliable predictors of successful clinical outcomes.