Lean manufacturing concept is to create more value for customer by eliminating non-value addingrnactivities. Manufacturing firms operating in the presentsâ€™ rapidly changing and highly competitivernmarket have been pressured to improve all aspects of operational performance; quality andrncustomer response time and reduce cost of manufacturing. This research sought to examine therneffect of lean manufacturing practices on the operational performance of Ethiopian army Adamarngarment industry. The study guided by three objective; To determine the extent to which Ethiopianrnarmy Adama garment industry have adopted lean manufacturing practices, To establish the effectrnof lean manufacturing practice on operational performance of Ethiopian army Adama garmentrnindustry and finally the challenge faced by the industry when implementing lean manufacturing.rnThe study adapted a descriptive research design with the use of cross-sectional data. The targetrnpopulation was 297 Ethiopian army Adama garment industry employees from differentrndepartments, from this 121 respondents were used as sample. A questionnaire was used to collectrnprimary data. Out of the 121 research instruments distributed, 107 questionnaires were collectedrnand analyzed representing 88.4% response rate which was considered adequate. Data analysisrnwas done with the use of SPSS which produced descriptive statistics and regression models usedrnto present the results in tables and figures. The finding indicated that Ethiopian army Adamarngarment industry not well practiced lean manufacturing. It was also clear that Ethiopian armyrnAdama garment industry had not seen an improvement in their operational performance. Therneffect of lean manufacturing on operational performance of Ethiopian army Adama garmentrnindustry was tasted against two operational performance metrics; product quality and on timerndelivery. The outcome showed that lean manufacturing practices; Just in Time, Total qualityrnmanagement, Total productive maintenance, and Human resource management are positivelyrnrelated to operational performance. Moreover, lean manufacturing practices have been employedrnto a small extent and have negatively affected operational performances to a large degree. Thernstudy also established that the most challenge in adopting lean manufacturing was lack of properrntraining/inadequate human resource development and the least was lack of resources. The studyrnrecommended that the Ethiopian army Adama garment industry should concentrate on all aspectsrnof lean manufacturing practice to bring superior operational performance. Implementationrnprocess of these strategies should be given top priority during formulation of organization policies.