Abstract: A study on the effects of different levels of potassium nutrition on the diurnal variation of the photosynthetic rate of flag leaves of wheat (Triticum aestivum cv. Anlong 8903) planted in the fields showed that from flowering stage to middle grain filling stage (May 1-20), the potassium content in the plants and leaves increased with increasing levels of potassium nutrition. Potassium content in the plant and leaves reached maximum levels that were 2.7 times the control at application rates of 82.5 kg potassium·hm-2, whereas at higher application rates of 330 kg·hm-2, potassium levels were lower (about 10%-30％). During the course of grain filling, the potassium content of the plants increased but that of the leaves decreased. From flowering stage to middle grain filling stage, the diurnal variation of stomatal conductance and of net photosynthetic rate revealed two maxima at about 10∶00 and 15∶00 because of the strong intensity of sunlight, high temperature and relatively low humidity. The maximum at 10∶00 is higher than that at 15∶00, with a minimum at 14∶00. However, the diurnal variation of the intercellular CO2 concentration (Ci) showed a contrasting tendency with two minima at about 12∶00 and 15∶00, and the former was lower than the latter. The maximum Ci level between the two minima was most pronounced at flowering stage but less pronounced at the middle grain filling stage. The results showed that increasing the potassium nutrition in the soil can increase stomatal conductance and net photosynthetic rates, and reduce the loss of photosynthetic CO2 fixation caused by photosynthetic midday depression by 10％-37% thereby raising the wheat output. The correlation between potassium content and intercellular CO2 concentration was not significant. The results of this study indicate that potassium additions of 165 kg K2O·hm-2 is the most economically efficient and optimal strategy for increasing net photosynthetic rates and thus wheat yields.