Sidareja Primary Canal of Sidareja Irrigation Area was built to irrigate 9635 hectares of agricultural land. Currently, the condition of the canal is undergoing some physical damage. The objective of this study was to determine the lost water discharge and efficiency value of the canal. The method used to analyze water loss and efficiency is to use the inlet and outlet discharges. The data used in this study are primary data in the form of flow velocity data using a float, wet cross-sectional area. It also uses secondary data in the form of a network scheme, evaporation data from the evaporation pan (Ep), soil data for the coefficient of soil seepage (C) and to determine the percolation price (P). From the calculation results, the average water loss that occurs in the Sidareja Primary Channel is 1.3802 m3/sec, water loss due to evaporation is 0.00604 m3/sec, water loss due to seepage is 2,207x10-6 m3/sec, and water loss caused by percolation is 4,531x10-3 m3/sec, and water loss caused by other factor is 1,3696 m3/sec. The average efficiency of the canal is 63,70%.