A cross-sectional study was performed to assess the status of full immunization of children under 1 year old in Mong Cai and certain related factors. The results showed that in 210 children in Mong Cai district, Quang Ninh province vaccinated during 2013 and 2014, 65 percent of children were fully immunized and 35 percent of children were not fully vaccinated. The drop-out rate between DPT1 and DPT3 was 28 percent. Psychological concern about adverse events following immunization (AEFI) directly affected the coverage of children's full immunization (48 percent of mothers did not vaccinate fully their babies), followed by postponed vaccination due to reserved indications of health workers (33.3 percent), anxiety and selected other vaccines (18.7 percent). Mothers living in city had lower fully immunized practice for their children than those living in the countryside and island by 3.19 times. Lower education level, official occupation and proper knowledge of mothers were some related factors to fully vaccinated practice for children under 1 year old. The study indicated direct influence of some AEFI cases to children's immunization coverage. Thence it was suggested that communication on immunization safety for mothers, caregivers and communities must be strengthened.