A cross-sectional study was done in Dong Da district, Hanoi capital in 2011 to determine the prevalence of Rickets, breastmilk Vitamin D concentration and related factors in infants aged 1-6 months. A total of 186 infants aged 1- 6 months and who were breast fed, and their mothers were randomly selected and invited to participate. Dianogsis of rickets based on clinic symptoms and blood analysis. Mother's breastmilk was collected for analysis vitamin D3 concentration by High-performance liquid chromatography (HPLC) method. Age, sun exposure of infants and related informations was evaluated by interviewing mothers. Results showed that, the prevalence of rickets of infants was 38,2 percent, increasing with age, lowest in infants aged 1-2 months (17,4 percent), highest in infants aged 5-6 months (56,7 percent). Mean of breastmilk Vitamin D concentration was 44 IU/100ml, prevalence of mothers with low breastmilk Vitamin D concentration was 60,4 percent. Factors associated with rickets were mothers on diet (OR= 5,74, p0,001), mothers did not take Vitamin Dtcalcium supplements during pregnance (OR