The ΔTmax, defined as the maximum of temperature difference between the two probes which implant adjacently in the upper and lower sap wood of the tree stem or root, is an important parameter for measuring sap flow by the Granier technique. The key for computing ΔTmax is determining a time step. The sap flow in the taproot of Caragana korshinskii from April to October in 2008 was measured continually and the environmental factors were recorded simultaneously in this study, and data from 30 sunny days were chosen for analysis. The results showed that ΔTmax was observed most frequently in the period from 22:00 to 6:00, and was higher in the 4 hours prior to and after midnight（with a frequency of 5263%）. The highest value occured at 0:00. The time of occurrence of ΔTmax was not significantly correlated with environmental factors and the driving force for sap flow was the water potential difference between leaf and root rather than the environmental factors. The ΔTmin, defined as the minimum of temperature difference between the two probes, occurred most frequently between 10:00 and 16:00, with the highest value at 14:00. There was a local minimum in ΔTmin at 14:00 when the solar radiation and the potential evapotranspiration（PYmax and ET0max, respectively）achieved their peak values. This was attributed to stomata regulation on sap flow at noon. Solar radiation was a key environmental factor triggering stomata regulation at noon. The suitable time step in Fs’ computing by Granier technique is one week.