The effects of heat stress on activities of photosystem Ⅱ in leaves of Actinidia arguta (Sieb.et Zucc.) Planch.ex Miq.(Kuilü,a cultivar) were studied by chlorophyll a fluorescence transient and JIP-test.The results showed that:(1)F_v/F_m of leaves did not significantly change at 35-48 ℃,but decreased at 52 ℃.(2)With increase of leaf temperature,the relative variable fluorescence at J and I steps of fluorescence transient (V_j and V_i) significantly decreased,but increased at 52 ℃,the relative variable fluorescence V_k increased gradually.(3)With the increase of heat treatment time,the V_j and V_i increased gradually,but Ψo decreased gradually,indicating that the acceptor side of PSⅡ was damaged severely.In summary,activity of the donor side and the acceptor side of PSⅡ was inhibited by high temperature.The donor side was more sensitive to heat stress.JIP-test parameters can evaluate effectively activity of photosystems Ⅱ in leaves of A.arguta under different temperatures.