Abstract -- Knowing the threshold rainfall conditions is essential for the warning system as well as the evacuating process in areas prone to debris flows. The initiation of rainfall-induced debris flow is examined in this study based on both theoretical analysis and field data. Hydrological parameters, such as rainfall patterns, rainfall intensity, rainfall duration and total rainfall, of 29 debris flow events are carefully evaluated. The threshold curve proposed by Caine (1980) is valid for the debris flows prior to the 921 Chi-Chi Earthquake only. Since many landslides and slope failures occurred during and after the 921 Earthquake, the amount of total rainfall triggering debris flows is considerably reduced. Consequently, the threshold conditions should reflect the change of the geological parameters. In this study, the threshold conditions for debris flows of both landslide type and mobilized type are evaluated based on field rainfall data. For landslide-type debris flows due to crack seepage, both the total rainfall and the rainfall intensity are the dominant factors. For landslide-type debris flows due to uniform seepage, the total rainfall with respect to time is the controlling parameter triggering the debris flows. While the value of peak rainfall intensity determines the occurrence of mobilized-type debris flows.