Flashflood and rainfall-induced landslides have been a serious threat to people in mountainous areas of Thailand. In-depth understanding of the process of soil moisture changes in soil slope during heavy rainfall is therefore important in developing early warning systems for such hazards. Past records of rainfall prior to slope failure shows that the rainfall total in excess of 150 to 300 mm would tend to initiate localize slope failure and large scale debris flow respectively. A study has been carried out to investigate soil-water characteristics curves of undisturbed soils from slopes which experienced large scale debris flow as well as localized slope failure in Thailand. New miniature tensiometers developed at Kasetsart University were employed to monitor the suction changes in these tests and in the field. The basic properties of the soils indicate that most soils from debris flow areas in Thailand are silty and of low plasticity(ML, SM, CL). The mechanism of slope failure is also demonstrated using simple infiltration model and slope stability equation. The wetting front traveling, soil characteristics and drainage condition at the bottom of the slope appeear to be important factors. Application of these test results for early warning system for flashflood and landslide disaster is finally demonstrated based on some simplifying assumptions.