Abstract: A coupled model of photosynthetic rate (Pn) and transpiration rate (Tr) was developed to simulate the three-dimensional distribution of leaf instantaneous water use efficiency (WUEI) in apple canopy. For convenience, WUEI was expressed as the ratio of net Pn to Tr. The experiment was conducted in a ‘Fuji’ apple (Malus domestica Borkh. cv. ‘Fuji’) orchard during the 2011~2012 growing seasons. The simulation was shown that WUEI increased rapidly with increasing CO2 concentration and relative humidity (RH); but decreased with increasing temperature and leaf water potential (Ψl). The diurnal course of WUEI in the canopy showed a double-peak curve on clear day. The average WUEI on clear days was 2.98 mmol?mol-1 which was about 42% higher than cloudy days. The study indicated that the sensitivity of WUEI to microclimatic factors and Ψl can be systemically assessed by the coupled model.