Pometia tomentosa is a dominant uppermost canopy tree in Xishuangbanna tropical seasonal rain forest. In this paper, the photosynthetic characteristics of the three layers of P. tomentosa canopy were measured with Li-6400 portable photosynthesis system, and the results showed that P. tomentosa had the characteristics of high light saturation point (1 000 - 1 500 micromol x m(-2) x s(-1)) and low compensation point (7.7 - 15.3 micromol x m(-2) x s(-1)). Its leaf photosynthesis had the adaptive responses when the high irradiance changed. PAR was the main factor affecting the diurnal changes of net photosynthesis. In December, the leaf grew well, and had a high ability of photosynthesis, with the diurnal change of P. for upper layer exhibiting single-peaked pattern and the maximum value being 8.9 micromol CO2 x m(-2) x s(-1). April was the alternative period of old and young leaves, and the ability of photosynthesis was dropped, with the diurnal change of P(n) for upper layer following double-peaked curve and the maximum value being 4.3 micromol CO2 x m(-2) x s(-1). The highest net photosynthesis rate was observed in July, with the diurnal change of P. for upper and middle layers exhibiting single-peaked pattern and that for lower layers being double-peaked. Short-term enriched CO2 concentration could evidently increase the photosynthesis of P. tomentosa, while the stomatic conductance and transpiration would be inhibited. When the CO, concentration increased from 400 micromol x mol(-1) to 800 micromol x mol(-1), water use efficiency (WUE) was increased by 50% - 100% in dry season, but rather low in wet season.