One of the most useful SAR data utilization is the natural hazard monitoring because SAR enable to acquire the data even under the storm or the darkness. The purpose of this study is easily to understand the flood monitoring of JERS-1 SAR by visualized method. The study area, which is annually covered by flooding water for a long period, is situated at the lower reaches of the Mekong River. This condition, however, is convenient to JeRS-1/SAR observation because of having 44-day repeat cycle for the tageted area. All of eight scenes, which were obtained from July 5, 1997 to May 9, 1998, were used in this study. On the poweer images the flooding areas are shown as the dark part, which means low back-scattering coefficient. The process of this method is as follow; 1). Register the two images using more than 10 Ground Control Poins (GCPs). 2). Select the pixels less than -13.4dB. 3). Subtract the Digital Number (DN)between the registered pixels. 4). Assign the rainbow colors to each pixel derived from the subtraction. 5).Detect the flooding area on the colored images. Ground truth was carried out to verify the result of image analysis. This study is showing that L-band, HH polarization microwave of JERS-1/SAR is effective to identify the flooding area and, even 44-day interval. This visuzlized method is clearly indicating that the flooding water does not directly come from the river bank but gradually from upstream and spread to the huge low land area.