Background: Vaccination is recognized as an important immunization tool which is used in preventing and eradicating communicable diseases. Even though an increase in global vaccination coverage, many children in the world particularly in low resource countries still are left unvaccinated and it remains to be a public health problem. So, the main objective of this study was to identify determinants of incomplete vaccination among children 12-23 months of age in Semen Bench district, Bench Maji Zone, Southwest Ethiopia, 2018. Methods: A community based unmatched case-control study was undertaken among randomly selected children aged 12 to 23 months and with a total sample size of 312 (104 cases and 208 controls). Simple random sampling was used to get representative cases and controls. Data was collected using a structured questionnaire and analyzed using SPSS version 21 statistical software. Bivariate and multiple logistic regression analyses were done to identify independent factors for incomplete immunization status of children. P values <0.05 with 95% confidence level were used to declare statistical significance Results: From the total of selected 104 cases and 208 controls, all of the cases and controls participated in the study with a response rate of 100. Based on the multivariable analysis the incomplete immunization status of children was significantly associated with having no antenatal care visits, [AOR=8.3(95%CI; 1.87-36.91], home delivery [AOR=4.5(95%CI; 1.411-14.27], having no postnatal care visit [AOR=4.2(95%CI; 1.67-10.41], and inconvenient appointment time [AOR=3.44( 95% CI:1.65-7.16)]. Conclusion: Children of mothers having no antenatal and postnatal care, children of mothers having home delivery, children of mothers with an inconvenient appointment time, were significantly associated with incomplete immunization. To reduce the number of children with an incomplete immunization status, the district needs to consider specific planning for mothers with no ANC, and focus on promoting facility delivery and address concerns of inconvenient appointment time at a community level. This can be accomplished by integrating the immunization service to other elements of primary health care