OBJECTIVE To study the impact of maternal weight gain during pregnancy on the risk of infant obesity within 1 year old. METHODS A total of 785 infants who were born in Hefei and participated children medical care in one district health center and their mothers were chosen as the research subjects from September 2010 to September 2011. Three groups were classified by weight gain during pregnancy according to the percentiles: excessive pregnancy weight gain group of 126 pairs, adequate pregnancy weight gain group of 542 pairs and inadequate pregnancy weight gain group of 117 pairs. Mother's general demographic information was collected. The height and weight were measured when the infant was 42 days, 3, 6, 9, and 12 months of physical examination. Z score was calculated. The differences of Z score in different groups were compared and the RR values of different weight gain during pregnancy on infant obesity were computed. RESULTS The weight-for-age Z score (WAZ) of infant at 42 days 3, 6, 9 and 12 months in excessive pregnancy weight gain group were 0.23 ± 0.93, 0.25 ± 1.03, 0.23 ± 0.99, 0.28 ± 1.09, 0.26 ± 1.14, respectively, all higher than that of the corresponding age in adequate pregnancy weight gain group (-0.04 ± 1.02, -0.07 ± 0.99, -0.05 ± 0.98, -0.06 ± 0.97, -0.07 ± 0.95, respectively). The differences were statistically significant (all P values < 0.05). In excessive pregnancy weight gain group, infant body mass index (BMI) at 9 months ((18.01 ± 0.15) kg/m(2)) and 12 months ((17.66 ± 0.15) kg/m(2)) were higher than that of adequate pregnancy weight gain group ((17.63 ± 0.13) and (17.22 ± 0.15) kg/m(2), respectively). The differences were statistically significant (all P values < 0.05). Differences of infant Height-for-age Z score (HAZ) among three groups were not statistically significant (all P values > 0.05). Compared to adequate pregnancy weight gain group, RR (95%CI) value of infant obesity in excessive pregnancy weight gain group was 1.86 (1.14 - 3.03). CONCLUSIONS Excessive maternal weight gain during pregnancy increased the risk of infant obesity within 1 year old.