BACKGROUND: Maternal and neonatal mortality is the concern of global population especially in sub-Saharan Africa including Ethiopia. Improving the maternal and newborn health through proper postnatal care services under the care of skilled health personnel is the key strategies to reduce maternal and neonatal mortality. But post natal care utilization is still low and maternal and neonatal mortality is high. OBJECTIVE: The aim of this study is to assess postnatal care service utilization and associated factors among mothers who gave birth between July 2015 and January 2016in Bahir Dar Zuria, Northwest Ethiopia. METHOD: A Community based quantitative cross-sectional study triangulated with qualitative study was conducted among 708 mothers who gave birth between July 2015 and January 2016. Multistage sampling was .used to select study participants. A pre-tested semi-structured questionnaire and focus group discussion were used to collect data. The quantitative data were entered in EPI info version 3.5.1 and analyzed using SPSS version 20 .00 while the qualitative data were analyzed manually. Bi-variable and multivariable logistic regression model was fitted to identify factors associated with post natal care service utilization. Odds ratio with 95 % confidence interval was computed to determine level of significance. RESULT: Among 708 respondents 79.9%were between the age group of 20 to 34 with a mean age of 20.2± (4.8 SD). Fifty six percent and 40.7% of respondents were ,... involved in WHOA and graduated in HEP respectively. The study revealed that the level. of Post Natal Care service utilizations was 35.6%. Among determinate factors - of PNC utilization being involved in WHOA (AOR=11.26(6.41, 19. 79)), being graduated in HEP (AOR=5.05(2.88, 8.87)), number of ANC visit (AOR=5.14, 95%Cl=2.88, 9.17)), place of delivery (AOR=3.3, 95%Cl=1.92, 5.68)) and knowledge on PNC (AOR=16.74(9.08, 30.86)) showed statistical significant association. The most frequent reasons for not obtaining the service during FGD were being healthy, cultural and religious believers and lack of awareness.