Abstract Objective: To evaluate the association between body mass index (BMI) and pregnancy outcomes in women receiving intrauterine insemination (IUI) treatment． Design: Retrospective cohort study. Setting: University-affiliated hospital. Population: Six thousand four hundred and seven women undergoing 13,745 IUI cycles stratified by body mass index (BMI). Method: Cox regression was used to analyze the association between BMI and cumulative live birth across multiple IUI cycles. GEE was used to analyze the live birth rate per cycle. Main Outcome Measure(s): Cumulative outcomes for IUI cycles. Result(s): Compared with normal weight women (n = 4,563), underweight women (n = 990) had a lower cumulative pregnancy and live birth rate (20.71% vs 25.93%, 17.07% vs 21.61%, respectively), while overweight women (n = 854) had a higher cumulative pregnancy and live birth rate (31.97%, 26.58%). Adjusted for confounders, hazard Ratio (HR) for achieving live birth following up to a maximal of 4 IUI cycles was 0.8(95%CI: 0.67-0.95) comparing underweight with normal weight. In generalized estimating equation (GEE) analyses, low BMI was also associated with a lower per-cycle live birth rate 0.77(95%CI: 0.64-0.92) with adjustment for cycle-specific parameters, including ovarian stimulation, endometrial thickness and follicular diameter. Conclusion(s): Underweight is associated with poor IUI outcomes. Funding: Clinical Research Special Fund of Chinese Medical Association (NO.18010360765) and Xiamen Medical Advantage Subspecialty Construction Project (2018[296])