It is indicated from measurements that the lower limit of the area contain ing the relational points between decay time of secondary electrical field and apparent resistivity has directly intimate interval, inversely intimate interval and non-intimate interval. This suggests that the decay time has close relation ship with the apparent resistivity, and any neglect of the variation in apparent resistivity may lead to error in the search for water by IP method.In view of the interval characteristics of the relationship between decay time and apparent resistivity, we have employed a concept of "relative dacay time" as the water-bearing factor in water-search using IP method (the relative decay timeS_R＝ρS/_s).The significance of adopting this parameter lies in making prominent the water-bearing anomalies in inversely intimate interval and elimi nating anomalies without water caused by increase in polar distance in directly intimate interval. The relative decay time therefore has a comparatively high sensitivity in the investigation into concentration of underground water in mountain areas.The application of relative decay time technique to water prospecting in mountain areas with different hydrogeologic conditions has already yielded quite good results. In the groundwater engineering of the well-known Meigui Spring where the resistivity method and the commonly-used decay time method had difficulties in interpretation and inference, the relative decay time method has located water-rich area with remarkable accuracy. Since the completion of the project, a water supply of 60,000t/day can be obtained and a large-size agricul tural groundwater source area has been established.