Abstract The main purpose of this study is to investigate the relationships among ion concentration、electrical conductivity (EC) of seepage water and soil mass movement occurrence. The sites of this study were selected at Feng-Chiu and Shen-Mu in Nantou and Sindian in Taipei. Soil samples for the tests were collected from the watersheds of the debris flow potential streams of the sites. After the soil samples were sieved and prepared in the flumes in a manner of simulating the soil physical conditions in the field and Equal-Weight Replacement Method, the seepage flow and slope stability tests were then conducted to investigate the variation of the ion concentration and EC of seepage water while the soil slopes failed. Meanwhile, the ion concentration and EC of seepage water in the field was also observed and evaluated. The results showed that while the seepage water table gradually increased and a curtain time period before the soil slope failed, for the samples collected from Feng-Chiu and Shen-Mu in Nantou and Sindian in Taipei, the concentration of SO42- and EC of the seepage water at upstream of the mass movement did not change significantly. However, the concentration of SO42- and EC of the seepage water at downstream of the mass movement started to increase and reached to a maximum value while mass movement occurred. Keywords: seepage water, ion concentration, electrical conductivity, soil mass movement