Acclimation to excess light is required for optimizing plant performance under natural environment. The present work showed that the treatment of Arabidopsis leaves with exogenous H 2 O 2 can increase the acclimation of PSII to excess light. Treatments with H 2 O 2 also enhanced the capacity of the mitochondrial alternative respiratory pathway and salicylic acid (SA) content. Our work also showed that the lack in alternative oxidase (AOX1a) in AtAOX1a antisense line and the SA deficiency in NahG (salicylate hydroxylase gene) transgenic mutant attenuated the H 2 O 2-induced acclimation of PSII to excess light. It indicates that the H 2 O 2-induced acclimation of PSII to excess light could be mediated by the alternative respiratory pathway and SA.