Abstract Background: Sex workers have been among the populations most affected by HIV since the beginning of the epidemic more than 30 years ago. In both concentrated and generalized epidemics, HIV prevalence is considerably higher among sex workers than in the general population. There for information about the current level of knowledge and preventive practices of FSW is needed to set out preventive efforts. This study will identify gaps and provides clues for programmatic intervention. Objective: To assess comprehensive knowledge and preventive practice of HIV /AIDS among female sex workers in Bahir Dar, North west Ethiopia, 2015. Method: A quantitative community based cross sectional study triangulated with qualitative methods was employed among FSWs from December 05 to 25, 2015 in Bahir Dar City. The data were collected by face to face interview, in-depth interview and observation from 584 FSWs who were selected by multistage cluster sampling. Bivarate and multi variable logistic regression was employed to identify the association between dependent and explanatory variables. The association was considered statistically significant when P value is < 0.05. Result: A total of 584 FSWs were studied with response rate of 97.3%.The comprehensive knowledge of FSWs on HIV/AIDS was 34.1 %. The overall preventive practice of the FSWs was 58.2% in which 65.6% used condom always and consistently. Age (AOR= 1.71,95%CI= 1.05-2.79), educational level (AOR= 2.27,95%CI= 1.37-3.78) and working as peer promoter for organization (AOR= 1.51, 95%Cl=1.02-2.23) were significantly associated with comprehensive knowledge. Substance use for alcohol users (AOR= 2.25,95%CI= 1.21- 4.19) and for khat users (AOR=4.80 ,95%CI=' 2.50- 9.20) membership with organization working on HIV/AIDS (AOR=2.55,95% Cl=1.52-4.27) were identified as significant predictor variables for HIV preventive practice of FSWs. Conclusion & Recommendation: Among female sex workers, comprehensive knowledge on HIV/AIDS was low while their preventive practice is at moderate level. ~~~i~- Therefore, we recommend stakeholders to intervening on these high risk population group focusing on the raising comprehensive knowledge and preventive practice ofHIV/AIDS