Tolerance of photosystem 2 (PS2) to high temperature in apple (Malus domestica Borkh. cv. Cortland) leaves and peel was investigated by chlorophyll a fluorescence (OJIP) transient after exposure to 25 (control), 40, 42, 44, and 46 °C in the dark for 30 min. The positive L-step was more pronounced in a peel than in leaves when exposed to 44 °C. Heat-induced K-step became less pronounced in leaves than in peel when exposed to 42 °C or higher temperature. Leaves had negative L-and K-steps relative to the peel. The decrease of oxygen-evolving complex (OEC) by heat stress was higher in the peel than in the leaves. OJIP transient from the 46 °C treated peel could not reach the maximum fluorescence (Fm). The striking thermoeffect was the big decrease in the relative variable fluorescence at 30 ms (VI), especially in the leaves. Compared with the peel, the leaves had less decreased maximum PS2 quantum efficiency (Fv/Fm), photochemical rate constant (KP), Fm and performance index (PI) on absorption basis (PIabs) and less increased minimum fluorescence (F0) and non-photochemical rate constant (KN), but more increased reduction of end acceptors at PS1 electron acceptor side per cross section (RE0/CS0) and per reaction center (RE0/RC0), quantum yield of electron transport from QA - to the end acceptors (ϕ R0) and total PI (PIabs,total) when exposed to 44 °C. In conclusion, PS2 is more thermally labile than PS1. The reduction of PS2 activity by heat stress primarily results from an inactivation of OEC. PS2 was more tolerant to high temperature in the leaves than in the peel. and L.-S. Cheng, L. Chen.