Background: Postnatal care is a care given to a mother and newborn within 42 day of birth. It is the key strategy to avert maternal and child deaths. Worldwide above 60% of maternal deaths occur in the first 2 day after birth. In Ethiopia, maternal deaths are high but postnatal care utilization is still very low. Objective: To assess magnitude and factors affecting postnatal care service utilization in Yilmanadensa district. Methods: A community based cross-sectional study design that employs both quantitative and qualitative data collection methods were employed from November to December 2018. Multistage sampling technique was used to select 738 samples. Data were collected using structured questionaire and semi structure interview guide for quantitative data and qualitative studies, respectively. Data for the quantitative study were entered in to Epi Info version 7 and export to SPSS version 23 software while thematic framework analysis was applied to the qualitative portion. Bi variable and multivariable analysis were used to identify factors associated with outcome variables and a variable with P value of <0.05 was considered statistically significant. Results: The postnatal care service utilization was 17.2% (95% CI: 14.4% to 20%).Maternal education (AOR=5.77, 95%CI=2.28 - 14.56), time taken to the nearest health facility (AOR=1.6, 95%CI=1.10 - 7.30). place of delivery (AOR=3.31, 95%CI=1.27 - 8.64), total pregnancy (AOR=3.03, 95% CI =1.67 - 6.7), planned pregnancy (AOR=3.77, 95% CI=1.55 - 9.04), place of birth (AOR=3.31, 95% CI =1.27 - 8.64), Vaginal-bleeding (AOR=6.23, 95%CI=3.23 - 11.90), husband permission (AOR=12.5, 95%CI=1.17 - 13.30), eclampsia/preeclampsia (AOR=12.96,95%CI=6.01 - 27.92), cultural belief (AOR=2.4, 95%CI=(1.10 - 5.10) were significantly associated with mothers who utilize postnatal care service. From the qualitative results transportation (emergency) problems, cultural influence, poor source of information and missed perception for institutional delivery and postnatal care follow up were the main barrier to utilize. Conclusion: The postnatal care service utilization was 17.2% (95% CI: 14.4% to 20%). Maternal education, pregnancy induced hypertension, vaginal bleeding, planned pregnancy, number of lived child, distance to the nearest health facility, place of birth, cultural belief were factors affecting postnatal care service utilization. Key Words: postnatal care, puerperium, postpartum, comparative study.