OBJECTIVE Our study aimed to determine the relationship between maternal diabetes mellitus (MDM) and mortality and major morbidities for very preterm infants, as well as the effects of insulin-treated MDM, in the Chinese population. METHODS This retrospective cohort study included all preterm infants born at 240/7 to 316/7 weeks of gestation and admitted to 57 tertiary neonatal intensive care units participating in the Chinese Neonatal Network in 2019. All infants were followed up until discharging from the hospitals. RESULTS A total of 9,244 very preterm infants were enrolled, with 1,584 (17.1%) born to mothers with MDM. The rates of mortality or any major morbidity in the MDM and non-MDM groups were 45.9% (727/1,584) and 48.1% (3,682/7,660), respectively. After adjustment, the risk of mortality or any morbidity was not significantly increased in the MDM group (adjusted odds ratio [aOR], 1.07; 95% confidence interval [CI], 0.94-1.22) compared with the non-MDM group. Among MDM mothers with treatment data, 18.0% (256/1,420) were treated with insulin. Insulin-treated MDM was not independently associated with the risk of mortality or any morbidity (aOR, 1.01; 95% CI, 0.76-1.34) among very preterm infants, but it was associated with an elevated risk of severe retinopathy of prematurity (aOR, 2.39; 95% CI, 1.13-5.04). CONCLUSIONS While the MDM diagnostic rate for mothers of very preterm infants was high in China, MDM was not associated with mortality or major morbidities for very preterm infants. CONCLUSIONS · A total of 17% of very preterm infants in Chinese neonatal intensive care units were born to mothers with MDM.. · MDM was not related to mortality or major morbidities in very preterm infants.. · MDM treated by insulin was associated with severe retinopathy of prematurity..