Tonle Sap Lake, along with the Tonle Sap River, forms a unique hydrological system due to monsoon climate. The reverse flow of the Tonle Sap River during wet season results in intensive flooding that enlarges the lake over vast floodplain and wetland where covered with forests, shrubs, etc. Monitoring the floodplain is essential for water resources, natural disaster management and environmental conservation. Radar interferometry (InSAR) can be applied to monitor the water-level change in wetlands and floodplains. Flood monitoring around Tonle Sap using ALOS PALSAR interferometry is presented. To examine backscattering characteristics in the floodplains of the Tonle Sap, PALSAR polarimetric data were reviewed. For classification of scattering mechanism, we used PLR (HH-, HV-, VH-, VV- polarization) data. Entropy-alpha distributions are compared between northwestern and central part of the lake. Double-bounce is dominant in the northwestern part of the lake. HH-polarization InSAR pairs is effective to measure water level change specifically at the northwest edge of the lake. Radar measurement of the water level change shows similar pattern to the water level change of long-term ground measured data and altimeter data. However, it was not possible to measure continuous changes of water level by InSAR because of relatively long period of the data interval. In summary, PALSAR inteferometry is applicable to water level measurement specifically at the northwest floodplains of the Tonle Sap. A follow-up studies is required for measuring continuous water-level change in the Tonle Sap.