Abundances of study in different population have noted that obese cardiovascular disease (CVD) patients have a better prognosis than leaner patients, which refer to the phenomenon of obesity paradox. However, data on the association between body mass index (BMI) and mortality among Asian patients are limited, especially in patients with type 2 diabetes mellitus (T2DM). We investigate the association between BMI and all-cause mortality in Taiwanese patients with T2DM to define the optimal body weight for health.We conducted a longitudinal cohort study of 2161 T2DM patients with a mean follow-up period of 66.7 ± 7.5 months. Using Cox regression models, BMI was related to the risk of all-cause mortality after adjusting all confounding factors.A U-shaped association between BMI and all-cause mortality was observed among all participants. Those with BMIs <22.5 kg/m had a significantly elevated all-cause mortality as compared with those with BMIs 22.5 to 25.0 kg/m, (BMIs 17.5-20.0 kg/m: hazard ratio 1.989, P < 0.001; BMIs 20.0-22.5 kg/m: hazard ratio 1.286, P = 0.02), as did those with BMIs >30.0 kg/m (BMIs 30.0-32.5 kg/m: hazard ratio 1.670, P < 0.001; BMIs 32.5-35.0 kg/m: hazard ratio, 2.632, P < 0.001). This U-shaped association remained when we examined the data by sex, age, smoking, and kidney function.Our study found a U-shaped relationship between all-cause mortality and BMI in Asian patients with T2DM, irrespective of age, sex, smoking, and kidney function. BMI <30 kg/m should be regarded as a potentially important target in the weight management of T2DM.