Objective To investigate the iodine status of lactating women and their infants during the first 4 month postpartum in the regions with optimal iodine nutrition.Methods Thirty pairs of lactating women and infants were followed up for 16 weeks after delivery. Urine samples were collected from mothers and infants at weeks 4, 8 and 12 and matal breast-milk samples were collected at weeks 4, 8, 12 and 16. The contents of iodine were measured accordingly.Results Over the period of 12 weeks, the median urinary iodine concentration (UIC) of lactating women was 110.1μg/L (from 78.3μg/L to 138.8μg/L). Breast milk iodine concentration (BMIC) was 171.8μg/L (from 125.4μg/L to 261.1μg/L) over the period of 16 weeks. There was significant difference between BMIC and UIC in lactating women (P<0.001). There was no correlation between UIC and the BMIC in lactating women. The median UIC in infants was 211.8μg/L (from 141.5μg/L to 283.1μg/L). No differences were observed among UICs of infants over the three sampling times. Infant UIC was correlated with BMIC, but not matal UIC. A significant difference was observed among BMICs over the four sampling times (P<0.001). A decline tendency was found in BMIC with time after delivery (R2=-0.545,P<0.001). Conclusion: BMIC was decreased in the first 16 weeks in the lactating women with an optimal iodine status. So adequate supplementation of iodine is recommended for lactating women.