Abstract: Using long time series (1961-2012)observational hourly rainfall data in Liaoning province, hourly 0.1°×0.1° resolution merged precipitation products over whole China from 2008 to 2013, NCEP reanalysis data from 2000 to 2012 and high resolution meso-scale numerical model WRFV3.3.1, the characteristcs of the diurnal variation of heavy rain in Liaoning province were summarized, and the mechanisms have been analyzed and numerically examined. The results are as follows. (1) The (heavy) rain peak inland of Liaoning province mostly happens from afternoon to midnight, while that inshore mostly happens in the morning. There is a secondary peak for inland plain stations from wee hours to morning. (2) There is a close relationship between the behavor of precipitation diurnal variation and the geographical environment in Liaoning province. The main reason of the rain peak in the afternoon inland and in the forenoon inshore is the local circulation diurnal variation caused by difference of thermodynamic character among the mountainous area, Northeast plain and the ocean. The rain peak inland from afternoon to midnight is caused by the updraft of the mountain-plain local circulation, while the inshore rain peak from wee hours to forenoon is caused by the updraft of the ocean-land local circulation. (3) The numerical test for inland stations without mountains shows that the daily accumulated rainfall was not affected, but the precipitation diurnal variation was obviously affected. After removing the mountains, the rain peak was advanced to noon and the peak value became smaller. The numerical test for inshore stations when altering ocean to land shows that the daily accumulated rainfall was also not affected, and the precipitation diurnal variation was again obviously affected. After altering ocean to land, the rain peak of inshore stations occurred at night instead of in the morning. In summary, the result of numerical test further illustrates that the large terrain of mountain and ocean is the main factors causing the precipitation diurnal variation in Liaoning province.