Objective To evaluate the effect of iodine supplementation on improvement of developmental quotient (DQ) at the critical period of infant brain development.Methods Pregnant,lactating women and infants less than 3 years old were supplemented with iodized oil in Linxia Hui Autonomous Prefecture(Linxia Prefecture) Gansu Province in 2006-2010.Before and after the intervention(2006,2007-2010),five townships were randomly selected in the north,the south,the east,the west and the center of eight counties(cities) of Linxia.One village was chosen from each of those townships and 20 infants,20 pregnant women and 20 lactating women were randomly selected in each village(insufficient was made up from the neighboring villages).Urinary iodine(UI) level of the infants,pregnant and lactating women were determined by arsenic cerium catalytic spectrophotometry.DQ value of infants was measured before and after supplementation of iodized oil in 2006 and 2010.UI value of pregnant,lactating women and infants was monitored every year after iodine supplementation.Results Before iodine supplementation(2006),the median UI level of pregnant,lactating women and infants was 89.28,84.85,107.3 μg/L,respectively.After iodine supplementation,the medians UI level in 2007,2008,2009 and 2010 were,respectively,pregnant women:136.0,187.8,118.2,175.8 μg/L; lactating women:135.2,159.8,187.5,163.5 μ g/L; infants:139.6,174.7,190.7,168.4 μg/L.Before iodine supplementation,the DQ value of infants was 92.8 ± 16.3,and the average score of gross motor,fine motor,adaptive capacity,language and social behavior was 93.7 ± 20.0,91.4 ± 20.0,92.4 ± 19.0,90.3 ± 20.0,96.4 ± 22.1,respectively.After iodine supplementation,the DQ value of infants was 104.3 ± 13.8,and the average score of gross motor,fine motor,adaptive capacity,language and social behavior was 104.8 ± 21.5,104.1 ± 17.2,104.8 ± 16.1,99.9 ± 19.1,108.0 ± 22.7,respectively,which were higher than that before iodine supplementation (t =-10.43,-10.77,-13.78,-14.28,-9.96,-15.33,all P ＜ 0.01).Conclusions Iodine deficiency at the critical period of brain development can affect the intellectual development of infants and young children at all functional areas of intelligence.Iodine supplementation at the critical period of brain development can prevent mental retardation caused by iodine deficiency disorders.