Background: Family planning allows people to attain their desired number of children and determine the spacing of pregnancies. It is achieved through use of contraceptive methods and the treatment of infertility. Objective: To assess determinants of Long acting and permanent family planning methods utilization among married women in Ilu Aba Bor zone, Oromia, southwest Ethiopia 2016. Methods: Institutional based case control study was conducted. Simple random sampling technique was used to interview total of 700 (233 cases and 467 controls) using interviewer guide questionnaire for data collection. Data were entered to epi data version 3.1 and analyzed by SPSS version 23.0 statistical software. Descriptive statistics and logistic regression were computed to identify and see the relative effect of predictors on the outcome. Result: In this study 700 women were included (233 cases and 467 control group). Majority of the cases 145 (62.2%) have ever used long acting and permanent family planning methods. Women with Number of alive births two or more are 10 times more likely to use long acting and permanent methods than who had less than two, AOR=10.32 95% CI (3.69-28.89) while women with family size of 3-4 and five or more are 90.7% and 82.9% less likely to utilize LAPM with AOR=0.093 95% CI (0.021-0.422) and AOR=0.171 95% CI (0.042-0.696) respectively than who had family size of two or less. Additionally, women who respond on the decision who can made on the number of birth you want to have as wife, both and God are identified factors of LAMP utilization with AOR=4.79 95% CI (1.37-16.76), AOR=5.637 95% CI (1.69-18.83) and AOR=26.78 95% CI (2.43-295.17) respectively. Women who ever shifted from long acting to short acting are 84.3% less likely to utilize LAMP than women who ever shifted from long acting to short acting family planning methods with AOR=0.157 95% CI (0.063-0.386). Conclusion: According to the findings of this study; number of alive births, family size, decision maker on the number of births and type of method shift are statistically significant predictors of utilization of long acting and permanent family planning methods.