Body mass index (BMI) has a J-curve relationship with mortality, but the physiological mechanisms underlying its effects are complex and paradoxical. Understanding whether early cardiac structural changes, specifically left ventricular mass (LVM), modulate the relationship between BMI and all-cause mortality is crucial for public health evaluation.To investigate the mediating role of LVM, in the relationship between BMI and all-cause mortality.The Chin-Shan Community Cardiovascular Cohort (CCCC) study was a prospective cohort consisting of participants from one community in Taiwan in 1990-1991. The inclusion criteria were adult participants in CCCC with complete data for BMI, and LVM from echocardiography measurements. Deaths were ascertained by hospital-based medical records and official death registry database. Cox regression models were applied to determine the association between BMI, LVM and all-cause mortality. The mediation was quantified using the Delta Method and confirmed via the Bootstrapping method (N=1,000 replicates).A total of 2,918 participants with the mean (standard deviation) age 54.2 (11.8) years and 54.1% women were included. Their mean (standard deviation) BMI was 23.51 (3.36) kg/m^2. In the median follow period was 22.4 years, 984 deaths developed. BMI significantly decrease the all cause-mortality risk, with the hazard ratio [HR]= 0.946 and 95% confidence interval [CI], 0.924-0.969. Meanwhile, LVM significantly increase the all cause-mortality risk, the HR (95 % CI) was 1.004 (1.003-1.005). The analysis revealed a statistically significant suppression mediation: the total effect of BMI on the hazard of death was estimated to be -0.021 [-0.042, -0.0004]. However, a significant positive indirect effect 0.0271 [0.011, 0.043], suggests that LVM mediates the association.This suppression effect highlights the "obesity paradox" in that BMI's overall benefit is actively counteracted by its detrimental effect on LVM. Public health efforts should consider LVM reduction alongside weight management.The study indicates that the protective direct effect of BMI is suppressed by an increased risk of death mediated through higher LVM.