- The aim of this study is to analyze high-frequency variability and diurnal cycle of precipitation during summer monsoon (June-September) for the period of 2000-2009 over Thailand, using TRMM satellite data (3B42 V6) at fine scales (0:25 x 0:25 and 3 hourly). Empirical Orthogonal Functions (EOFs) and Harmonic Functions were used to describe the dominant patterns of diurnal precipitation variability under the different phase reversals of the Asian Summer Monsoon. The EOF analysis showed that the first two EOF modes could explain almost the total variance of the climatological diurnal precipitation during the summer monsoon season. The 1st EOF mode accounted for 68.7% of the total variance, while the second EOF mode explained 21.8% of the total variance. The diurnal precipitation cycle of the EOF1 ¡-11 mode reached the maximum in the afternoon and evening [15.00-18.00 LST], whereas it showed the early-morning minima [06.00-09.00 LST]. This diurnal pattern is consistent with those generally found over most land areas, characterizing by high amplitude due to the marked contrast of heat, moisture and momentum between land, ocean and atmosphere. The diurnal precipitation variability in the EOF2 mode was marked by nighttime maximum [21.00-00.00 LST] and gradual decline to reach minima in the afternoon [15.00-16.00 LST]. The diurnal cycle as seen in the EOF2 mode may reflect local characteristics of orographical and geographical effects. On the basis of grid-by-grid Harmonic analysis, it was found that the summation of the first four orders of harmonic function could perfectly represent the averaged diurnal precipitation cycle over Thailand. The first and second orders of harmonic function could explain the variance of climatological diurnal precipitation cycle during the summer monsoon season by 79.6% and 16.3%, respectively. The amplitude of the first and second orders of harmonic function accounted for 50.3% and 17.9% relative to the long-term averages of 3-hourly precipitation during the summer monsoon season. The results indicate that summer precipitation over Thailand was dominated by diurnal (24 hours) cycle. When comparing the diurnal precipitation variability over Thailand in the different phases of the Asian Summer Monsoon, the results based on both the EOF and Harmonic analysis could not show the clear different patterns of diurnal precipitation cycles in terms ¡-12 of variance, phase and amplitude during the strong, normal and weak years of the Asian summer monsoon. The short period of data used which spanned only 10 years may be one possibility causing unclear patterns of precipitation with respect to different phases of the Asian Summer Monsoon. For further work, the diurnal cycle of precipitation in the different phases of El Nino- Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Madden Julian Oscillation (MJO) needs to be analyze to better understanding of high-frequency variability of precipitation over Thailand especially its impacts in the forms of climate-related disasters in the context of anthropogenically-forced warmed climate and enhanced regional hydrological cycle.