Abstract: Relationship between diurnal changes of net photosynthetic rate (Pn) and the impact factors in leaves of Stevia rebaudiana Bertoni were studied. Diurnal changes of Pn and environmental factors (photosynthetic available radiation and temperature and relative humidity) and physiology factors (stomatal conductance and transpiration rate and intercellular CO2) in leaves of Stevia rebaudiana Bertoni chicory were measured using LI-6400 portable photosynthesis system. The relationship between Pn and environmental factors were analyzed by regression analysis and path analysis. The results showed that the curve of diurnal changes of Pn was demonstrated two peaks in clear day and appeared midday depression at noon and the stomatal conductance decreased. All those were due to high photorespiration which was caused by high light intensity and high temperature.