Abstract: Responses of photosynthesis and PSII photochemistry activities to chilling treatment under low irradiance, a simulated unfavorable condition of sunshine greenhouse in deep winter, were investigated in two cultivars of pepper with different chilling sensitivity. Chilling treatment (15℃/5℃, day/night) under low irradiance (100 μmol•m-2•s-1, 12h photoperiod) for 1~7 days resulted in less decreases in carboxylation efficiency (CE), stomatal conductance (Gs), photosynthesis capacity and maximum regeneration of RuBP in chilling-tolerant pepper “Jiamusi” than in chilling-sensitive pepper “Longjiao No. 6”. Chilling treatment under low irradiance for 1~7 days had little effects on Fv/Fm, ΦPSⅡand qP in Jiamusi. But such treatment caused decreases in Fv/Fm, ΦPSⅡand qP in longjiao No. 6 after 3 days. The Gs of Longjiao No. 6 was highly sensitive to chilling treatment under low irradiance, and the maximum Gs decreased by 66.41% in Longjiao No. 6, but only by 15.20% in Jiamusi when treated 1 day. In Longjiao No. 6, Pn could not be detected when treated 1 day, but in Jiamusi it could be until treated 5 days. Chloroplast pigments contents decreased in leaves of both two cultivars of pepper during chilling treatment under low irradiance. Chla / Chlb ratios changed little, and Car / ChlT ratios reduced in leaves of Jiamusi, but Chla / Chlb ratios and Chla / Chlb ratios increased in leaves of Longjiao No. 6 under low temperature and low irradiance. During the treatment, activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) were higher in Jiamusi than in Longjiao No. 6. Chilling treatment under low irradiance for 1~3 days enhanced the activities of SOD, APX and GR simultaneously in Jiamusi. But GR activity was not enhanced as the activities of SOD and APX in Longjiao No. 6 during the treatment.