OBJECTIVE: To evaluate the associations of adjunctive vitamin E (Vit E) and coenzyme Q10 (CoQ10) supplementation with embryological and pregnancy outcomes in patients with diminished ovarian reserve (DOR) undergoing IVF/ICSI-ET. METHODS: In this retrospective observational study, 949 patients with DOR undergoing IVF/ICSI-ET were included. According to adjunctive treatment before controlled ovarian stimulation, patients were categorized into the Control (n=338), Vit E+CoQ10 (n=260), CoQ10 (n=224), or Vit E (n=127) groups. Demographic characteristics, ovarian reserve indicators, stimulation parameters, embryological outcomes, and pregnancy outcomes were compared. Multivariable Poisson regression models with a log link and robust standard errors adjusted for demographic characteristics, ovarian reserve indicators, stimulation parameters, fertilization method, and treatment cycle initiation year were used to assess associations between supplementation regimens and embryological outcomes. A 2 × 2 exposure-based interaction model including Vit E use, CoQ10 use, and their interaction was applied. RESULTS: Baseline characteristics and stimulation parameters did not differ significantly among groups. The distribution of treatment cycle initiation year also did not differ significantly among the groups (P = 0.325). The numbers of transferable and top-quality embryos differed significantly across groups and were highest in the Vit E+CoQ10 group. Pairwise comparisons showed that Vit E+CoQ10 was associated with more transferable embryos than Control and Vit E, while the CoQ10 group had more transferable embryos than Control and Vit E. The Vit E+CoQ10 group also had more top-quality embryos than the other groups. After multivariable adjustment, combined supplementation and CoQ10 supplementation were associated with increased numbers of transferable embryos, whereas combined supplementation did not differ significantly from CoQ10 alone. A positive statistical interaction between Vit E and CoQ10 was observed for the number of top-quality embryos. Clinical pregnancy, live birth, and early miscarriage rates did not differ significantly. CONCLUSIONS: In patients with DOR undergoing IVF/ICSI-ET, CoQ10 supplementation was associated with a higher transferable-embryo count, whereas combined Vit E+CoQ10 supplementation was associated with a higher top-quality-embryo count. However, these embryological differences were not associated with improved pregnancy outcomes.