To determine the characteristics of altitudinal dependence in rainfall in the mountainous area of the upper Chao Phraya river basin in northern Thailand, one and a half years of rainfall data from 13 sites, installed by the GAME-T project, distributed throughout the 3, 853 km² of the Mae Chaem watershed were analyzed. It was found that the altitudinal increase in rainfall was obvious in the two wet seasons in 1998 and 1999 with the increment in 1999 being more than 2.5 times greater than that in 1998. The altitudinal increase in rainfall was also obvious in the dry season from 1998 to 1999, but the increment was smaller than in the two wet seasons. It is not the rainfall intensity, but the rainfall observed hours which causes the difference of altitudinal increment. These relationships were analyzed over a range of coefficients of variation (CV) for daily rainfall to understand how the spatial scale of the rain event affects the altitudinal increase in the rainfall. It was found that the altitudinal increase in the mean daily rainfall (MDR) was detected at both the high and low CV ranges. The increment for MDR with a low CV, which corresponds to a relatively larger spatial scale rainfall, is greater than that for high CV rainfall events for two wet and one dry seasons. For the large spatial scale rainfall, this tendency was due to the altitudinal increase in the duration for the large spatial scale rain in the wet seasons. In the dry season, however, this tendency was due to the altitudinal increase in the mean rainfall intensity.