Rainfall is an important parameter in Hydrology. It significantly contributes to hydrological modelling of the regional scale. Limitation of rain gages and miss recording rainfall data may force Cambodia to face the difficulties of watershed management and study. This report identifies the performance of a rainfall interpolation method called Inverse Distance Weighting and do the map of its spatial distribution. By using FORTRAN programing language, the interpolation method is formulated and conducted in daily time step 168 stations distributed around the country for each 25 km2 (5 km×5 km) grid in an area of 181 035 km2 of Cambodia. The data record in daily time step is practical for 29 years period from 1985 to 2013. The result shows that the variety of rainfall changes significantly from 1000 mm to nearly 4000 mm (3900mm) with an annual average rainfall equal to 1447 mm, the maximum rainfall value 1861 mm and the minimum rainfall value 1120mm. The performance of IDW is somewhat acceptable. 56 most records stations are selected for partial cross validation of IDW performance of the three month period from 1st August to 31st October 2000, the most severe flood time. Root Mean Square Error evolves from a small error of 0.5 mm to a high error of 34.3mm. For all of cases, this study illustrates that the IDW will be sufficient for Cambodia whenever the data distribution of the point observation is dense enough in the whole country. Therefore, there must be a small scale study for dense rain gage area. Furthermore, others methods such as Geostatistical method might be recommended for interpolation and study its performance in this case of the sparse rainfall stations.