In Mongolia, significant soil erosion is thought to be caused by overgrazing and unsuitable management. Soil erosion also make desertification, a key issue of environmental concern. However, very few field data are available on overland flow mechanism, soil erosion processes and amount and the desertification processes. The purpose of our research project (2001-2006) is to study and assess the state and the causes of soil erosion through the monitoring of overlandflow flow and soil erosion.The study site locate 60-100 km east of Ulanbaatar. One site is Kherlen
Bayaan Ulaan (KBU), and the other site is Baganuur (BG). The drainage area of the watershed is 7.1 ha and 8.0 ha, and relative height is 105 m and 160 m, respectively. Parshall flumes and sediment traps were installed at the outlet of the catchments. Also we established 2 hillslope plots (50m x 50 m) in each area in which one is surrounded by fens. Discharge flow plots were monitored by Parshall flumes,and tensiomters, raindrop impact sensors, wind velocities were also recorded. About 50 Soil cores were sampled in each catchments and Cs-137 analysis were conducted. The runoff peak in 2003 in KBU is greater than BG. Through the Cs-137 analysis, long term soil erosion is very significant throughout the catchment is KBU, but deposition area were detected downslope of the catchment and lower overall erosion rate in BG. These data suggest the overlandflow erosion is significant in the study area, but importance of surface erosion varied between place to place.