Abstract: In order to reveal the daily variation process of sap flow rate and stem diameter and its impact factors, field experiments were conducted to collect data of summer maize in middle growth period at Daxing Station, and then the relationship between daily variation process of sap flow rate and stem diameter, the relationship of sap flow rate and environmental factors, as well as the relationship of the microvariation of stem diameter with soil moisture were analyzed. Results showed that the curve of daily sap flow rate variation process was unimodal, and significantly affected by solar radiation, vapor pressure, wind speed and other meteorological factors. A linear regression equation was obtained through analysing the data of sap flow rate and meteorological factors, which may provide a basis for prediction of leaf transpiration of maize using meteorological data. Moreover, the daily change of stem diameter was significantly different in the daytime and at night, i.e. shrank in the daytime and recovered at night. The maximum of daily stem diameter decreased with decreasing soil moisture, forming a linear relationship between them. So crop water deficit conditions can be diagnosed by using the data of stem microvariation on the basis of this relationship.