In general, sex ratio at birth is relatively stable with the standard biological 104-106/100. Nowadays, Vietnam is facing an increased risk of sex ratio at birth. Objective: Describe the sex ratio at birth and some factors that related to the sex ratio at birth in Dong Da district, Hanoi from 2008 to 2011. Method: A followup study at Dong Da district. Results: Overall, the sex ratio at birth in this period was 123/100. It had changed over the years but generally the higher the bio-equivalence (104-106/100). Sex ratio at birth was highest in 2010 (143/100) and lowest in 2008 (113/100). Sex ratio at birth was high at the first delivery (124/100) and to become highest at the third delivery (187.5/100). Sex ratio at birth in group of mothers less than 25 years was highest 138.8/100. Mothers as workers and hand labor had sex ratio at birth (144.2/100) higher than the other groups. The nearly poor households had sex ratio at birth (137.1/100) higher than the other groups.