Objective Our purpose was to evaluate the association between hemoglobin concentration (Hb) and cognitive ability of children at 4-6 years of age in 21 counties/cities in China.Methods A total number of 7331 children born during 1993-1996 were randomly selected from 21 counties or cities in Hebei,Jiangsu and Zbejiang provinces,Hb concentration of children were measured followed by three tests including full-scale,verbal and performance intelligence quotient (IQ) test performed by Chinese-Wechsler Intelligence Scale for Children,one year later.Results There were a 0.7 point difference in mean verbal scale IQ(P=0.144) and a 0.9 point difference in both mean performance and full-scale IQ (P=0.055 and 0.079,respectively) between anemia and non-anemia groups.Compared with children with non-anemia,children with anemia were 1.3-fold more likely to score poorly in verbal IQ and operational IQ (95%CI:1.1-1.6,1.1-1.5,respectively) and 1 A-fold more likely to have had poor scores in full-scale IQ(95%CI:1.2-1.6) after controlled for children's gender,age at intelligence test,region,parity and mother's IQ,education level,occupation.Participants were divided into 5 sub-groups according to Hb concentration of every 20 percentile.Verbal IQ scores of the lowest(Hb＜110 g/L),moderate (117 g/L≤Hb＜122 g/L) and the highest Hb concentration groups (Hb≥130 g/L) were 90.6±18.1,94.0±17.6 and 91.0±16.4,respectively.Performance IQ scores were 102.2±15.7,104.6±14.8 and 100.5±14.9,respectively.Full-scale IQ scores were 95.94±17.3,99.0±16.4 and 95.2±15.6,respectively.Children with both low and high hemoglobin levels did poorly in all intelligence tests than children with moderate Hb concentration (P＜0.001).After controlling for confounding factors,children with the lowest concentration were 1.4-fold more likely to have had poor verbal and performance scores than children with moderate Hb concentration (95% CI:1.1-1.7,1.1-1.8,respectively) and 1.5-fold (95% CI:1.2-1.8) more likely to have had poor full-scale scores than those with moderate Hb concentration.The association between high Hb concentration and low IQ scores disappeared in the multivariate model.Conclusion Low Hb concentration might have adversely affected children' s cognitive development.