People are faced with shortages of freshwater for consumption and irrigation during the dry season and the early part of the wet season1 as well as during the dry spell within the wet season. This paper assesses rice water use in Chrey Bak river catchment located in the Rolea B’ier district, Kompong Chhnang province - Cambodia using CROPWAT model. The study of rice crop water requirement is important not only for irrigation system design, irrigation scheduling but also for water resources management. The main purpose is to calculate rice water requirement by CROPWAT model, firstly in order to compute data into the CROPWAT to analyse crop evapotranspiration to understanding the consumption of water in the catchment and secondly to make maps of rice crop water requirement and rice irrigation requirement to provide information on water demand, land characteristic and land use change across the lower and upper regions of the Stung Chrey Bak catchment. Depend on the intensive study of the paper, monthly meteorological weather data recorded from 2010 to 2013 and soil characteristic data were used to obtain the results. The rice crop water requirement (CWR) data were calculated for each parameter and the obtained rice irrigation requirement (IR) data were compared. CWR and IR was calculated using CROPWAT model. The results of water requirement by model show that all dry season rice consume water corresponding to general rice (450 mm-700 mm) for the total growing period. However rainy season rice give outcomes of water use too much over the range. Thus, the result can be acceptable for rice cultivation.