Since 2010, the EPI project has implemented "5 in 1 vaccine" (including 5 antigens: DPT, Hepatitis B and Hib) for immunization for children in the whole country. Aiming at understanding the safety of this new vaccine, the authors carried out the study to describe its AIFI. The study design was cross-sectional survey with sample size was 225 mothers and their children who have been fully immunized with this vaccine from June to December, 2010 in Khanh Hoa province. The results showed that: No severe AEFI found, 46.7 percent of vaccinated children had some common reactions. 41 percent had low fever, 12,4 percent had pain and swell at injection site, 18,6 percent cried over 1 hour. Frequency of AEFI was lower and lower from the first dose to the third dose of vaccine. There was no relationship between the weigh at birth, breastfeeding status and AEFI.