The effect of water deficit on sap flow of longkong trees was studied by using heat-pulse method. An experiment was carried out in a glasshouse over 30-day period. There were 3 treatments: 1) daily watering or control, 2) 5-day interval watering or moderate water deficit (MWD) and 3) 10-day interval watering or high water deficit (HWD) with 4 replications. Twelve longkong trees (3-year old) were used. Each tree was grown in a 30L polybag filled with soil mixture. It was found that leaf water potential, stomatal conductance and Fv/Fm (at 10 day interval) in the MWD and HWD continuously decreased through the experimental period, and they were significantly different from those of the control at the end of the experimental period. Sap flow in the MWD and HWD treatments were consistently low around day 16 and day 13 after starting the experiment. Concomitantly, stomatal conductance of HWD rapidly decreased from day 10, and it was significantly different from those in MWD and control treatments. This implies that water deficit caused stomatal closure leading to the reduction of transpiration. Hence, it appears that water deficit causes the decrease of sap flow in longkong tree, and this incidence depends on the severity of water deficit.