By using a Li-6400 portable photosynthesis system, this paper measured the maximal photosynthetic capacity (P(max)) and water use efficiency (WUE) of the seedlings or saplings of 150 species of tropical montane rainforest in Hainan Island. The results showed that among the test 150 species, Quercus bambusifolia (Fagaceae) had the highest P(max) (13.27 micromol x m(-2) x s(-1)) and Macaranga denticulate (Euphorbiaceae) had the highest WUE (5.99 mmol x mol(-1)), while Decaspermum cambodianum (Myrtaceae) had the lowest P(max) (1.35 micromol x m(-2) x s(-1)) and Sapium discolor (Euphorbiaceae) had the lowest WUE (2.4 mmol x mol(-1)). The average P(max) was the highest in Moraceae but the lowest in Myrtaceae, and the average WUE was the highest in Symplocaceae but the lowest in Moraceae. However, there were no significant differences (P > 0.05) among the families. No significant differences (P > 0.05) in P(max) and WUE were also observed among arbors and shrubs. The P(max) value increased significantly (P < 0.05) with the widening of families' geographical distribution range, in the order of narrow distribution families < medium distribution families < wide distribution families, while no significant differences were existed among their WUE (P > 0.05). The P(max) value also increased with the widening of species' geographical distribution range, in the order of endemic species in Hainan Island < tropical species < tropical-subtropical species < tropical-subtropical-temperate species, but WUE was opposite to P(max), namely, in the order of endemic species in Hainan Island > tropical-subtropical species > tropical species > tropical-subtropical-temperate species.