Background: Exclusive breastfeeding (EBF) has been practiced all over the world as the best cost effective way of feeding infants. In Ethiopia, breastfeeding is a norm and essential for child survival. However, the pandemic HIV/AIDS and the recognition that HIV positive mothers can transmit the virus to their babies through breast milk precipitated a terrible public health dilemma in countries like Ethiopia where incidences of HIV is high. Objective: To assess the EBF practices of HIV positive mothers and its determinants in a selected health institution of West Oromia (Ethiopia). Methods: Institution based descriptive cross-sectional study was conducted through quantitative and qualitative methods in selected health institutions with ART & PMTCT facilities in west Oromia during January to May 2014. A total of 119 HIV positive mothers with their young infants visiting the health institutions were recruited for the study. Results: This study show that the practices of EBF, mixed feeding and replacement feeding were at 72%, 24.6%, and 3.4% respectively. Regarding determinant factors only work place was found to be the only predictor of the practices of EBF among HIV positive mothers (AOR=0.348, 95% CI: 0.121 to 0.995). Conclusion: Of the assumed determinant factors considered in this study, HIV positive mothers who work far from their home were found to be 0.348 times less likely to practice EBF than those mothers who work near their home (AOR=0.348, 95% CI: 0.121 to 0.995).