The natural 13C abundance (δ13C) of plant leaves collected from fields in Thailand and Philippines (Asian Monsoon tropics) was analyzed, and changes in the δ 13C values of C3, and C4 plants in wet and dry seasons were characterized. In Thailand, the δ 13C values of C3 plants were -29.2 ± 1.04 ( mean ± std. deviation ) 0 in July and August ( wet season ) and -28.6 ± 1.05 0 in Feb. and March (dry season) : there values are not significantly different, whereas the values of C4 plant were -12.7 ± 0.56 0 in the wet season and -14.5 ± 0.68 0 in the dry season (P < 0.01, t-test ).