Objective: To evaluate the impact of elevated body mass index (BMI) on short- and long-term outcomes of in-vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) treatments. Design: Retrospective cohort study. Setting: Teaching hospital. Population: Overall, 7229 patients undergoing IVF/ICSI fresh cycles and subsequent frozen embryo transfer cycles during 2014-2020. Methods: The patients were divided into normal (18.5–24.9 kg/m2) and high BMI (≥ 25 kg/m2) groups. Subgroup analyses were performed based on the boundary of 38 years old. Multivariate analysis was used to determine whether BMI was associated with live birth rate (LBR) or cumulative live birth rate (CLBR). Main Outcome Measure: Ovarian response, pregnancy outcomes, and safety for both mother and fetus. Results: For younger women (< 38y), CLBR was significantly reduced in the high BMI subgroup compared to the normal BMI control (73.7% vs 76.8%, p = 0.008) and was accompanied by fewer retrieved oocytes and available embryos. Meanwhile, the incidences of cesarean section (92.9% vs 87.1%, p < 0.001), hypertensive disorders of pregnancy (6.7% vs 3.1%, p < 0.001), fetal macrosomia (4.7% vs 2.8%, p = 0.002) and birth defects involving cleft lip and palate (0.4% vs 0.1%, p = 0.030) were significantly higher than the normal BMI group. However, no such differences were observed among older women (≥ 38y). Multivariate analysis revealed that high BMI was a risk factor for CLBR (OR = 0.837, 95% CI: 0.729–0.96). Conclusions: Elevated BMI has a greater adverse impact on younger women.