The floodplain area was largely influenced by the fluctuation of water level and seasonal change around Tonle Sap Lake. In the wet season, the lacustrine landform and vegetated areas were covered with water, and vice versa in the dry season. Monitoring of this area provided significant information required for human activities and sustainable development. In order to monitor the annual seasonal changes, backscattering coefficients of ALOS PALSAR have been measured for seven objects interval 46 days during flood period from 2007 to 2010. Dual-polarization (HH+HV) were used to improve the accuracy of the results. The purpose of paper aims to monitoring the marshes flood extent based on the backscattering coefficients. The results indicated that the backscattering coefficients decreased, while the water levels increased in lowland. However, the backscattering coefficients changed at upland due to local flooding and cultivating plan, harvest and growing process of crop.