Background: Globally adolescents are facing a range of sexual and reproductive health problems like unwanted pregnancy, unsafe abortion, STI including HIV. In addition, uptake of reproductive health services among adolescents remains low. Several factors are implicated for poor uptake of contraceptive methods among adolescent girls. However, adolescents’ mother knowledge and attitude towards contraceptive methods on adolescent contraceptive utilization has not been adequately studied. Objective: To assess the prevalence of contraceptive use and associated factors among unmarried female adolescents. METHODS: A community based cross-sectional design with both quantitative and qualitative methods was conducted in Butajira South Central Ethiopia with a sample size of 491 unmarried female adolescent and their mothers. A multi stage sampling technique was used. Interviewer administered pre-tested structured questionnaire and interviewer topic guide was used to collect data for quantitative and qualitative respectively. The data was entered into Epidata and exported to STATA for analysis. The quantitative data analysis includes descriptive, bivariate and multivariate logistic regression analysis. Content analysis was use to analyze the qualitative data. Result: A total of 491 unmarried female adolescent & their mothers were approached of which 450 gave consent to participate in the study giving a response rate of 90.4%. The prevalence of contraceptive use among sexually active adolescent was 43(15.2%). A unit increase in adolescents’ mothers knowledge score on contraceptive methods is associated with an increase in the odds of ever use of contraceptives among adolescent by 1.32 [AOR=1.32: (95%CI: 1.121.46)]. Furthermore ever use of contraceptive was found to be significantly associated with adolescents age [AOR=3.8 (95% CI: 1.33, 10.8)] and adolescents knowledge on contraceptive [AOR=1.23 (95%CI: 1.12-1.47)]. Our qualitative findings showed that majority of adolescents’ mother do not advise use of contraceptives by their adolescent girls. Conclusion and Recommendation: Prevalence of contraceptive utilization among unmarried sexually active adolescent in BHDSS was low and maternal knowledge of modern contraceptive was found to be significantly associated with adolescent’s contraceptive use. Therefore Policy makers and programmers should strengthen different programs to improve contraceptive utilizationamongadoelscents.