Assessment of rainfall-induced shallow slope failures is very important in order to reduce damages to infrastructures system, and lives of people nearby hazardous areas. Recently, the common effective tool for prediction of shallow slope failures has been critical rainfall concept because of its simplicity and faster assessment. However, this concept has been developed based on statistical approach from historical slope failure data, in which many factors those might trigger shallow slope failures are disregarded. This paper attempts to assess the probable factors triggering shallow slope failures. A set of parametric study was performed via finite element model to evaluate the effect of periodical condition of rainfall on initiation of shallow slope failures. The time to slope failures (D_f ) under various rainfall intensity I_f was collected. The relationship between and D_f for initiation of shallow slope failures (ID thresholds) was plotted. The results indicate that periodical rainfall plays an important role on failure state of shallow slope. The D_f under assigned I_f decreases with a shorter period between two rainfall events. It also results in the fitting parameter represented interception of ID thresholds. Furthermore, the general failure mechanism of shallow slope is introduced in this paper.