Leaves from annual young grape plants (Vitis vinifera L. cv. Jingxiu) were used as experimentalmaterials. The ultrastructural characteristics of mesophyll cells in chilling-treated plants after heat acclima-tion (HA) and in heat-treated plants after cold acclimation (CA) were observed and compared using trans-mission electron microscopy. The results showed that slight injury appeared in the ultrastructure of meso-phyll cells after either HA (38℃ for 10 h) or CA (8℃ for 2.5 d), but the tolerance to subsequent extremetemperature stress was remarkably improved by HA or CA pretreatment. The increases in membrane perme-ability and malondialdehyde concentration under chilling (0℃) or heat (45℃) stress were markedly inhib-ited by HA or CA pretreatment. The mesophyll cells of plants not pretreated with HA were markedly dam-aged following chilling stress. The chloroplasts appeared irregular in shape, the arrangement of the stromalamellae was disordered, and no starch granules were present. The cristae of the mitochondria were dis-rupted and became empty. The nucleus became irregular in shape and the nuclear membrane was digested.In contrast, the mesophyll cells of HA-pretreated plants maintained an intact ultrastructure under chillingstress. The mesophyll cells of control plants were also severely damaged under heat stress. The chloroplastbecame round in shape, the stroma lamellae became swollen, and the contents of vacuoles formed clumps.In the case of mitochondria of control plants subjected to heat stress, the outer envelope was digested andthe cristae were disrupted and became many small vesicles. Compared with cellular organelles in controlplants, those in CA plant cells always maintained an integrated state during whole heat stress, except for thechloroplasts, which became round in shape after 10 h heat stress. From these data, we suggest that thestability of mesophyll cells under chilling stress can be increased by HA pretreatment. Similarly, CA pretreat-ment can protect chloroplasts, mitochondria, and the nucleus against subsequent heat stress; thus, thethermoresistance of grape seedlings was improved. The results obtained in the present study are the first,to our knowledge, to offered cytological evidence of cross-adaptation to temperature stresses in grapeplants.