In the submerged macrophyte-dominated Gonghu Bay of Lake Taihu, this paper studied the epiphyte biomass and its effect on the rapid light curves (RLC) of Potamogeton malaianus and Myriophyllum spicatum during the vigorous growth seascon (from July to August) of submerged macrophyte by using a submersible and pulse-amplitude modulated fluorometer (Diving-PAM). Hie results showed that the epiphyte biomass attached to leaves df M. spicatum was significantly higher than that attached to leaves of P. malaianus, but the ratio of heterotrophic biomass to autotrophic biwnass in epiphytic axnmunities attached P. malaianus was not different frran that attached M. spicatum. Hiotosynthetic fluorescence characteristics, such as the photosynthetic coefficients, the half saturaticm point of light intensity and maximal relative electron transport rate of P. malaianus and M. spicatum with epiphyte were lower than those the same plant species without epiphyte. Tlie results indicated that epiphyte decreased photosynthesis submerged plants.