The responses of photosynthetic rate, transpiration rate and water vapor transfer coefficient (D) to light intensity, leaf-air vapor pressure difference (leaf-air VPD) and leaf water content were measured by the chamber method. The assimilation chamber was designed to be independently control environmental factors such as light intensity, temperature, humidity and wind speed. The responses of photosynthesis, transpiration and D to light intensity were shown almost the same pattern when leaf temperature, leaf-air VPD and wind speed were maintained constant. When only leaf-air VPD was changed under constant conditions of other factors, the relationship between photosynthesis and transpiration 2 was negative an had no correlations. When only leaf water content was changed by detaching a leaf blade from the plant under constant environmental conditions, the responses of photosynthesis, transpiration and D to leaf water content had almost the same trend. The relationships between photosynthesis and transpiration were summarized as follows: when transpiration was directly proportional to D, it was positively correlated with photosynthesis; when transpiration was inversely proportional to D, it was negatively correlated with photosynthesis; and when transpiration had no relation to D, it had no correlation with photosynthesis. It was concluded that photosynthesis was affected directly by D rather than by transpiration.