Abstract: The diurnal change of net photosynthetic rate (Pn) and physiological-ecological factors of photosynthetic “midday depression” in eggplant (Solanum melongena L.) were studied. The results showed that, the diurnal changes of Pn of leaves in eggplant showed a double peak curve (there was a significant “midday depression” at noon), whether of Erming or Kuaiyuan; seedling stage, flowering stage, or fruits growing stage; top leaves, middle part leaves or bottom leaves. But the extent and the time of midday depression were different. The stomatal conductance (Gs), transpiratory rate (Tr), intercellular CO2 concentration (Ci), and air CO2 concentration (Ca) of eggplant leave were decreased when midday depression was happened, but photosynthetic active radiation (PAR) and leaf temperature (TL) were maximum. The reasons of midday depression of eggplant leaves were stomatal factors.