Children represent the most vulnerable age group in any community. In Sudan they have higher morbidity and mortality rates. Mothers' health seeking behaviors are major determinants for improving child health and knowledge about the factors influencing such practice is very crucial for designing effective intervention strategies. This studyaimsto study factors affecting health seeking behaviors for illness in under five children among the mothers in Khartoum state, Sudan. It was Across-sectional study usingbothquantitative and qualitative methods, Four hundreds and ninety five (495) mothers of those who visited the vaccination units of 33 public health centers were interviewed. Four focus group discussions were held. Chisquare test and regression analysis were used to determine the association between different factors and health seeking behaviors. The results showed that Eighty percent of mothers (80%) seek health care for their ill children. Of them, 70% went immediately to heath facility after disease occurrence. The person who took the decision of going to health facility (p-value=0.000) and work time for the health facility (p-value=0.001) were found to be significantly associated with mothers' seeking behavior using bivariate and multivariate analysis. Since the person who took the decision of going to health facility is the most determinant for mothers' seeking behavior, awareness rising among mothers about the importance of going to health facility immediately after disease occurrence can improve child health outcomes.