Abstract: Traditional singletree water consumption acquired by means of the scaling-up method from leaflet transpiration measurement, caused serious deviation to the very value. The whole tree water consumption by crown transpiration is closely approximated to the same of sap flow rate in the trunk sapwood which was measured by thermal dissipation probe (TDP) implanted in the trunk. The forest community water consumption can be acquired by scaling-up of singletree water consumption. Spatial-and-temporal sap flow velocity (SFV) fluctuation in the sapwood at lower and upper trunk position of Quercus variabilis and Pinus tabulaeformis, and other meteorological factors, such as solar radiation intensity, air humidity, air temperature, wind speed and soil temperature were measured in a 48 year-old mixed stand at an east hill slope, in the Forest Research Station of Beijing Forestry University in the West Mountains of Beijing (N 39°54′, E 116°28′) in May, 2001. The main elements of sap flow ascending force were transpiration pulling tension (which was determined by crown transpiration) and capillary tension (which was determined by the capillary diameter). Patterns of daily and diurnal SFV fluctuation of the two tree species were quite different. Diurnal SFV fluctuation of Q. variabilis appeared “ascend promptly in the morning (5:30 -10:00 am) and get the peak in the afternoon （13:00-16:00）, then descend slowly and reach the valley during the night”. SFV in upper trunk of Q. variabilis was much higher than that in the lower trunk, and the peak and valley appearing time in the upper trunk were much more early than that in the lower trunk, too. SFV and its rhythm of P. tabulaeformis were different to that of Q. varaibilis during the measuring season with the daily crest appeared in the early morning (4:20-8:00) and went down to the valley in the mid-day (11:00-14: 20). Daily SFV peak height of P. tabulaeformis was only one fourth of Q. variabilis. Fluctuation of solar radiation intensity, air temperature, and air humidity appeared a similar rhythm to the SFV of Q. variabilis, and indicated that there was a close relation between SFV and the meteorological factors. Problems existed in the SFV measurements were discussed in this paper.