Studied are stress effects of surface increased ozone concentration on winter wheat photosynthesis in an open field in open top chambers (OTCs). Results show that results under the treatment of (100±8) nmol?mol-1 of ozone are compared to those from the control run (CK), indicating that from the heading to mature stage contents of chlorophylls a and b as well as the total increase first and then drop, but the carotenoid content and the value of value of chl a/b keep steadily rising and declinling, respectively, there is no significant decrease in the net photosynthetic rate and stomatic conductance during the early stage of heading but their drops increase gradually with prolonged smoking time. In the whole experiment the apparent quantum yield and activation per unit area and PSII maximum light quantum efficiency of winter wheat display no perceptible reduction while the transpiration rate, dark respiratory rate, light compensation point and maximal photosynthetic rate are suppressed more greatly. For the treatment of (150±8) nmol?mol-1 of O3 the leaf carotenoid content, dark respiratory rate and light compensation point are all well above those of the CK in all growth/development stages, the other parameters being significantly below. These results indicate that the increase in surface ozone concentration has its photosynthesis threshold range over 100-150 nmol?mol-1 and as the smoking time increases, and the degree at which the leaf photosynthesis is suppressed becomes intensified step by step.