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Abstract Introduction Conventionally, the recommended duration of adjuvant chemotherapy of colon cancer had been 6 months. The IDEA Collaboration suggested that shortening capecitabin and oxaliplatin (CAPOX) adjuvant chemotherapy may be possible. S‐1 and oxaliplatin (SOX) treatment is standard treatment in metastatic colorectal cancer in Japan. The aim of this study was to optimize treatment dosage and duration of adjuvant SOX in stage III colon cancer. Methods This trial was as open‐label multi‐center randomized phase II study. Patients with stage III colon cancer were randomly assigned to 3 months or 6 months of adjuvant SOX treatment in different doses: 130 mg/m2 (3 months) or 100 mg/m2 (6 months) of oxaliplatin. The primary endpoint was 3‐year disease‐free survival (DFS) and the null hypothesis for the primary endpoint was that the 3‐year DFS was ≤72% in each arm and was tested with a one‐sided significance level of 10%. Results Eighty‐two patients were assigned to the 6 months arm and 81 to the 3 months arm. The 3‐year DFS was 75.0% (80% CI 67.95–80.72, p = 0.282) in the 6 months arm and 76.9% (80% CI 70.1–82.38, p = 0.171) in the 3 months arm. Treatment completion rate and relative dose intensity (RDI) were higher in 3 months than 6 months arm. The adverse events (AE) were similar in both arms. Conclusions The 3‐year DFS was not significantly superior to null hypothesis in both 3 months and 6 months arms for the stage III colon cancer. Primary endpoint was not achieved. The SOX regimen was not feasible in long‐term outcomes.