As one of the most important afforestation and greening tree species in the semi-arid area of China, Pinus sylvestris var. mongolica has been widely used in development of urban forests. Study on the transpiration of Pinus sylvestris var. mongolica and its response to environmental factors has essential theoretical significance and application value for urban forestry practices. Using the thermal dissipation probes (TDP) method, sap flow of Pinus sylvestris var. mongolica was monitored at the Hohhot Arboretum, Hohhot, Inner Mongolia, China from August to October in 2012, and environmental factors were measured simultaneously by using a portable automatic weather station and soil moisture sensors. Daily transpiration of trees varied with weather conditions. During sunny days, the transpiration rate curve showed a single peak, and the transpiration rate was higher than that on the cloudy or rainy days. The transpiration rate was not significantly correlated with the diameter of the tree trunk at breast height (P> 0.05). The main meteorological factors controlling the sap flow were solar radiation (r= 0.731, P< 0.01) and vapor pressure deficit (VPD) (r= 0.877, P< 0.01). The correlation coefficient between the sap flow and wind speed was significantly low (r= 0.518, P< 0.01). In addition, the soil water content did not influence the sap flow significantly. We concluded that VPD and solar radiation can explain 71% and 68%, respectively, of the daily sap flow change.