Irrigation scheme performance assessment is vital to evaluate the impacts of irrigation practices, to identify performance gaps and improve system performances. Therefore, this study was carried out to evaluate internal and external performance of Legemera small scale irrigation scheme. The irrigation scheme command area was132ha but during the study season, the irrigated area was 120ha. CROPWAT 8.0_model were used to analyze the data. Collecting primary and secondary data of the irrigation scheme has been carried out during the study. Primary data collection included canal water flow measurement, soil moisture one day before irrigation and one day after irrigation, depth of water applied to the field and field observations. Secondary data were collected from secondary sources that were Woreda Agriculture office, Woreda Metrological Station of the scheme. In order to evaluate the irrigation water use efficiency of farmers at field level, three farmers were selected from the scheme in relation to location (from the head, middle and tail end water users). The whole length of the canals in this scheme was concrete lined and have an average conveyance efficiency value for the main canal, the right side secondary canal and left side secondary canal was found to be 95.10%, 94.8% and 94.37% respectively. This shows the scheme performes well with respect to water conveyance capacity when it Compare with other similar studies put average convyance efficiency of well aintained and concrete lined 90%. However, Water application and storage efficiencies of the irrigation scheme were 50.20% and 22.14% respectively. Those result were poor when compared with application efficiency of properly designed furrow irrigation ranges from 55%-70%and that of the storage efficiency of 63% usually found in typical furrow irrigation systems. The computed water delivery capacity ratio result was 0.62% which shows that actual water delivered is less than the intended water volume and hence the design capacity is not a constraint to crop water demands. Based on computed relative water supply indicator result which was 1.17, water was not a constraint at farm level and higher amount of water was diverted to the scheme.