Abstract: Diurnal variation curve of net photosynthetic rate (Pn) of flag leaf of winter wheat in farmland of Tibet plateau was flat one or with one single peak, having hardly midday depression of Pn. Daily maximum of Pn was near to approach that observed in the east plain of China. The peak of the daily total amount of Pn is about 66%～96% of that in the plain. In Tibet plateau, the environmental conditions which enable the flag leaf of the plant to have Pn higher than 20μmol CO2·m-2·s-1 were the following : PFD was higher than 2000μmol·m-2·s-1, Ta was 25～29℃, ambient CO2 density was higher than 0.41mg·dm-3. Soil surface temperature was 18～23℃ ,and underground temperature (5cm depth) was 15～19℃. Probability at which suchlike factors combination occurs was not high. Ambient CO2 density was about one third lower than that in the plain. It restricted the daily total amount of Pn. Flag leaf of the plant in Tibet plateau had daily maximum of Pn approach similar to that in the plain, but its daily total amount of Pn was lower than that in the plain. Photosynthetic capabilities were high, but sunlight utilization efficiency was low.