Abstract:Background: In Ethiopia, a considerable proportion of childhood morbidity and mortality is due to vaccine preventable diseases. According to EDHS 2011, only 24% of children were fully vaccinated nationally. Mothers' knowledge, attitude and practice are key tools to decrease dropout rates and to prevent or control infectious diseases. Thus, this study was aimed to assess Knowledge, Attitude and Practice of mothers regarding infant immunization and their associated factors in immunization clinic in health centers at Addis Ababa, Ethiopia. Methods: Institutional based cross-sectional study was conducted from March 1 st to April 1 st , 2013. Multi stage sampling technique was used for participant selection. Participants were interviewed with structured questionnaire for different variables. Descriptive statistics and binary logistic regression analyses were performed during data analysis. Results: Only 55.0%, 53.8%, and 84% of respondents had good knowledge, positive attitude, and good practice towards immunization of infants, respectively. Maternal education (Adjusted Odds Ratio [AOR[= 1.781, 95% CI: 1.035, 3.065), respondents who had infants aged from 3-9 months (AOR=1.947, 95% CI:1.051, 3.607), 9-12 months (AOR =2.305, 95% CI: 1.216, 4.371) and mothers who gave births greater or equal to two times (AOR = 1.560, 95% CI: 1.087, 2.238) were significantly associated with knowledge of mothers regarding immunization of infants. Mothers' education (AOR = 2.160, 95% CI: 1.208, 3.864) and mothers who had infants' aged from 2-3 months (AOR = 2.014, 95% CI: 1.044, 3.883) were significantly association with favorable attitude towards immunization of infants. Good infant immunization practice was significantly associated with mothers who heard information about vaccination (AOR=1.784, 95% CI: 1.002, 3.176), mothers who know correctly the time when infants should begin immunization (AOR=2.240, 95% CI: 1.198, 4.192), know the number of sessions needed (AOR=1.772, 95% CI: 1.076, 2.918), know the time when infants should complete immunization (AOR=1.800, 95% CI: 1.123, 2.885) and place of delivery (AOR=23.829, 95% CI: 10.025, 56.639). Conclusions: Knowledge and attitude of mothers' about infant vaccination was not adequate. Despite inadequate knowledge and attitude of mothers towards infant immunization, 84.0% of mothers found in Addis Ababa had good practice of infants' immunization. Health education to promote knowledge and attitude based immunization practice is recommended.