Abstract: Evapotranspiration (ET) is an essential factor to estimate crop water use. It is also one of the major elements in soil water storage and water resource in a region. This study was accomplished to compare the yearly trends of ET among three main vegetations in Thailand, a rain-fed paddy rice field, cassava plantation and teak plantation. Bowen Ratio Energy Balance (BREB) technique was applied in this study to measure the ET and heat flux between ground surface and atmosphere. Penman-Monteith (PM) equation, recommended by FAO, was used to calculate reference crop evapotranspiration (ET0). Measurements were carried out during 1999-2003 in the both paddy rice field and teak plantation and during 2002-2003 in the cassava plantation. The results indicated that the amount of daytime ET during the rainy season in the paddy rice field and cassava plantation varied between 1 and 7 mm and in the teak plantation between 2 and 6 mm. The averages amounts of daytime ET in the rainy season were about 4 mm in all sites, although, the variations of ET were different. In the dry season, day time ET of the cassava plantation was around 2.7 mm, slightly lower than those of other sites. During the growing season, ET/ET0 varied in the paddy rice field between 0.4 and 1.2 and in the cassava between 0.3 and 1.2. In the rainy season, LE/Rn ratios of the cassava plantation and paddy rice field were around 70%, while LE/Rn ratio in the teak plantation was found to be around 73%. Long-terms trends of ET and ET/ET0 were observed in the main vegetations of Thailand characterized by tropical monsoon climate. Meteorological data were limited to only routine meteorological measurements in this region.