This study aims to analyze the effect of moderate-intensity swimming exercise which performed in the morning and at night on reducing body weight and Lee's obesity index in female mice (Mus musculus). This study used the true experiment method with the randomized control group post-test-only design. A total of 30 female mice (Mus musculus) at 3 months of age, and weighing 30 -40 grams (Lee's Obesity Index Value > 0.3) were divided randomly into three groups of 10, namely CTRL (n = 10, control without intervention), MISEM ( n = 10, moderate-intensity swimming exercise in the morning), and MISEN (n = 10, moderate-intensity swimming exercise at night). Body weight and Lee's obesity index was measured twice, namely preexercise and 4 weeks post-exercise. Then, data was analyzed using the Paired Samples T-Test with the Statistical Package for the Social Sciences SPSS version 21 software.The results show that the average body weight on CTRL between pre-exercise vs. post-exercise (33.60±2.17 vs. 39.20±3.82 gr (p-values=0.001)), )), and MISEN (33.40±3.47 vs. 26.00±2.26 gr (p-values=0.000)). Lee's average obesity index on CTRL between pre-exercise vs. post-exercise (0.39±0.02 vs. 0.44±0.02 gr/mm (p-values=0.000)), MISEM (0.40±0.02 vs. 0.42±0.01 gr/mm (p-values=0.000)), MISEN (0.41± 0.01 vs. 0.37±0.02 gr/mm (p-values=0.002)). Based on the results of the study, it was concluded that moderateintensity swimming exercise in the night was more effective in reducing body weight and Lee's obesity index than moderateintensity swimming exercise in the morning and control without intervention.