A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (gs) model that combines net photosynthetic rate (PN), transpiration rate (E), and gs, and also the leaf energy balance. The model was run to simulate the diurnal courses of PN, E, gs, photochemical efficiency, i.e., ratio of intercellular CO2 concentration and CO2 concentration over leaf surface (Ci/Cs), and leaf temperature (T1) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, gs decreased the most and E the least. The duration of midday depression of gs was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in gs. When air temperature exceeded a certain value, an increase in solar irradiance raised T1 and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (α). PN, E, and gs showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T1 under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in Ci/Cs at midday implies that stomatal closure is a factor causing midday depression of photosynthesis.