Moderate to vigorous physical activity is associated with improving insulin resistance in overweight and obese adults. However, effect of light physical activity on insulin resistance remains to be fully elucidated. The purpose of this study was to investigate the effect of light physical activity on insulin resistance in overweight and obese men. Thirty-seven overweight and obese middle-aged men (28.9 ± 1.8 kg/m², 51.0 ± 8.8 years) participated in this study. They took part in the lifestyle intervention (calorie restricted diet and exercise) for 12 weeks. Anthropometric parameters, fasting glucose, HbA1c, fasting insulin, and HOMA-IR were assessed at baseline and post intervention. Physical activity was objectively measured using a triaxial accelerometer at baseline and during intervention. Light physical activity (+17.7 min/day) and moderate-vigorous physical activity (+33.2 min/day) increased significantly, while body weight (-12.4 kg), fasting glucose (-9.5 mg/ml), fasting insulin (-4.2 μU/ml), and HOMA-IR (-1.1) decreased significantly. The change in light physical activity from baseline to during intervention were inversely related to change in fasting insulin (r = 0.18, P = 0.02) and change in HOMA-IR (r = -0.16, P = 0.03), after adjustment for several confounders. These results suggest that an increase of light physical activity improves insulin resistance in overweight and obese men.