The leaf structure of different bitter melon cultivars resistant to powdery mildew was compared and studied.They mainly included thickness of upper and lower epidermis;thickness of palisade tissue and spongy tissue;compactness and porosity of leaves structure;waxes content;leaf weight ratio;density of stoma and fuzz,etc.The result indicates that:(1)The waxes content of resistant cultivars is significant higher than that of susceptible one and significant negative correlation with disease index.Wax layer is an important structure barrier to resist and delay the pathogen invading.(2)The stomatal number and hair densities of abaxial leaves of susceptible cultivars are significant more than that of resistant ones.The stomatal number and hair densities of abaxial leaves are significant positive correlation with disease index.It indicates that the less stoma and fuzz,the more resistance.(3)The palisade and spongy tissue of resistant bitter melon cultivars arranged closely and in good order,but the leaf tissue section of high susceptible cultivars present lots of pores and hard to observe whole cells.(4)The thickness of leaves,lower epidermis and palisade tissue and leaves structure compactness of resistant cultivars is significant higher than that of susceptible ones,but the thickness of spongy tissue and porosity of leaf structure is opposite.To the powdery mildew resistance ,the leaf weight ratio count for little.Wax content,the stomatal number and hair density of abaxial leaves can be the reference designators to identify the resistance to powdery mildew for bitter melon.