We investigated the effects of vitamin B1 deficiency on the meiosis maturation of oocytes. Female Crl:CD1 (ICR) mice were fed a 20% casein diet (control group) or a vitamin B1-free diet (test group). The vitamin B1 concentration in ovary was approximately 30% lower in the test group than in the control group. Oocyte meiosis was not affected by vitamin B1 deficiency when the deficiency was not accompanied by body weight loss. On the contrary, frequency of abnormal oocyte was increased by vitamin B1 deficiency when deficiency was accompanied by body weight loss (referred to as severe vitamin B1 deficiency; frequency of abnormal oocyte, 13.8% vs 43.7%, P = .0071). The frequency of abnormal oocytes was decreased by refeeding of a vitamin B1-containing diet (13.9% vs 22.9%, P = .503). These results suggest that severe vitamin B1 deficiency inhibited meiotic maturation of oocytes but did not damage immature oocytes.