Abstract: To examine the effect of exogenous sucrose in protecting photosystem II (PSII) against salt and heat stress, PSII (BBY particles of PSII) prepared from the leaves of spinach (Spinacia oleracea) were treated with high temperature (30°C and 40°C) and salt solution (400 mmol.L-1 and 800 mmol.L-1 NaCl), and PSII activity was determined with the plant efficiency analyser (PEA). Salt and heat stress led to serious damage to PSII, inducing a marked decrease of PSII photochemistry (Fv/Fm). Under a combination of salt stress and high temperature, PSII activity was decreased more markedly than that under a single stress. The addition of different concentrations of sucrose (100-800 mmol.L-1) in storage buffer significantly alleviated the damage of high salt (400 mmol.L-1 and 800 mmol.L-1 NaCl) and high temperature (30°C and 40°C), or the combined stresses, to spinach PSII particles. Thus, exogenous sucrose significantly alleviated the damage induced by high salt and temperature to spinach PSII particles in vitro.