Growing season length is an important factor affecting energy, water, and carbon exchange at deciduous forests. However, our understandings about the extent of inter-annual variation in growing season length and its cause of deciduous forests in tropical region are still not enough, though the impact of modification of growing season length is likely to be critical especially in tropical region because of high radiant energy throughout the year. In order to clarify the extent of inter-annual variation in growing season length and its major cause of a teak plantation in Northern Thailand, we monitored radiative transmittance through the canopy and heat-pulse velocities of canopy trees for 7-year period. We found large inter-annual variations in the timings of leaf-out, leaf-fall, and the start and stop of transpiration. As a result, we revealed that the year-to-year variation in the length of canopy duration and transpiration period spanned about 60 days. It was also found that these large year-to-year variations can be explained by the differences in the timings of soil moisture changes caused by the differences in the timings of rainfall occurrence. These results imply a profound potential impact of modification of rainfall regime on canopy-atmosphere water and carbon exchange on annual time scale through the modification of growing season length.