The analysis of surface displacements and groundwater level at different locations in a sandy model slope subjected to artificial rainfall was conducted to establish a time prediction method of the onset of instability for rainfall-induced landslides. The analysis revealed that the relation between the surface displacement and the groundwater level can be modeled as hyperbolic. Time prediction based on the monitoring data was conducted based on the hyperbolic relation as following procedures: (1) to derive regression equation for the surface displacement ~ groundwater level relation and that for the time ~ groundwater level relation at any time before the failure, (2) to combine both equations to derive the time ~ inverse of the surface displacement velocity relation. This equation could simulate the actual time of failure of the model slope as well.