The different mechanisms of diurnal variation proposed in previous studies are tested using the abundant observational data during TOGA COARE. The preliminary findings are: (1) The stratiform precipitation lags the convective precipitation, suggesting that the effect of the mesoscale convective systems (MCS) life cycle is important to the midnight rainfall maximum, i.e., the stratiform component helps to shift the maximum toward midnight. This supports the MCS life cycle mechanism. (2) Our result supports the direct radiation-convection interaction mechanism and emphasize that the variation is caused by the stratiform cloud. (3) The current result does not support the day versus night radiation-subsidence mechanism. The vertical motion in the undisturbed region maybe partly forced by the disturbed region. (4) The current result does not support the large-scale radiative destabilization mechanism as the cause of the nocturnal maximum of total precipitation, although it may contribute to the evening maximum of the convective component of precipitation.