Abstract: Effects of all parameters influenced by the three levels of shading (0％ shading，HL；75％ shading，LL；40％shading．ML)on water use efficiency(WUE)of tomato leaves grown in pots under well-watered conditions were investigated in summer sunny midday．The results show that environmental factors influence WUE synthetically and complexly．especially，stomatal resistance and vapor pressure difference(VPD)．The stomatal resistance and the VPD increase by high evaporation potential because strong light and high temperature at noon at HL are more grea ter than those at ML and LL treatments．The increase in stomatal res istance can increase WUE，but the increase of VPD can decrease WUE．Daily variations of WUE under three shading levels show shallow “L” type，which are caused by the non-sym metry of diurnal variations of environmental factors and interaction of two mechanisms above．Compared with the non-shaded control，the average single leaf WUE of 40％-shaded plants increases by 22．9％ during the 12：O0～15：00 in summer sunny day，while the WUE for 75％-shaded plants reduces significantly by 28．1％．showing that moderate shading can enhance WUE of tomato leaves under sufficient water condition on summer midday．