Abstract: Investigations of field showed that Pteris ensiformis has the ability to acclimate to different light levels. This study investigated the ability of individuals of Pteris ensiformis to acclimate to low and high irradiance. Plants were cultivated in understory and gap of tropical primary rainforest for about 40 days. Individuals growing in understory indicated more number of leaves, bigger canopy width, higher chlorophyll contents and higher photosynthetic rates than that of plants growing in forest gap. Plants growing in understory also exhibited a greater efficiency in the photochemical utilization of absorbed light energy and lower ability to dissipate excess energy nonphotochemically, relative to the plants growing in forest gap. However, maximum photosynthetic rates were similar in both sets of plants, reflecting the higher efficiency of energy conversation in the understory-growning plants and an apparent saturation of photosynthetic capacity in the gap-growing plants. The latter may have resulted from the injury of the photosynthetic apparatus in addition to an increase in nonphotochemical dissipation of excess light energy. The higher capacity for harmless thermal dissipation of excess light energy should be beneficial in plants growing in exposed locations and subject to drought and nutrient stresses. Thus, the results expanded those plasticity in adjusting the photosynthetic apparatus to various light levels constitutes a valuable adaptation to growing in different light environments in the tropical rainforest.