Abstract: In order to investigate the effect of endophyte infection on its host plants, net photosynthetic rate， transpiration rate, water using efficiency and dry mass yield were determined using endophyte-infected (EI) and endophyte-free (EF) populations of Lolium perenne L. at different nitrogen levels. There were no significant differences between EI and EF populations in net photosynthetic rate，transpiration rate or water using efficiency during the experimental period but the cumulative value of diurnal photosynthesis was greater in EI populations than in EF populations both the medium and at high nitrogen level. No significant differences existed between EI and EF populations in dry mass yield of the herbage during the first or the second experimental period. During the third experimental period, the dry wass of the EI populations was significantly higher than the EF populations.