Abstract: Effects of water stress on ultrastruture of Malus leaves were studied with polyethylene glycol(PEG6000) treatment. The results were as follows: Under water stress, the contents of chlorophyll(Chl.), carotenoid (Car) in leaves decreased. Ultrastructure changes of mesophyll cell of Malus hupehensis Pamp. Reld.and Malus sieversii (Ledeb) Roem. were examined with transmission electron microscope. The observations showed that the ultrastructure changes occurred under water stress, the changes were consistent with drought-resistance.