To improve water management for increasing rice productivity, the study of crop water demand is very important. This study aims to estimate the water requirement of rice by using direct measurement method (water balance conception method). The observation was undertaken in a small experimental paddy field, which has an area of 2650m2, located in Cambodian Agricultural Research and Development Institute (CARDI) in Cambodia. For Phka Rumduol variety (120 days) in the rainy season (August 1 to November 27, 2013), the total rice water requirement is about 651mm which lost 448mm (69%) by evaporation, 165mm (25%) by transpiration and 38mm (6%) by percolation. For Chul’sa variety (100 days) in the dry season (February 1 to May 11 2014), the total rice water requirement is 604mm which lost 406mm (67%) by evaporation, 146mm (24%) by transpiration and 52mm (9%) by percolation.