Abstract: In order to explore the effects of cold-shock on the growth and flower bud differentiation of tomato seedlings under high temperature, tomato seedlings were subjected to coldshock treatments every day with 10 ℃  for 10 minutes in an artificial climate chamber. Tomato seedlings were treated with cold-shock at the first true leaf stage and the treatment lasted for 15 days. Tomato seedlings without cold-shock were used as control. At the fourth true leaf period of tomato seedlings, five plants were randomly sampled and the growth characteristics and the ultrastructure changes of mesophyll cell of tomato seedlings were examined. The flower bud differentiation process of tomato seedlings was observed at the periods of the second, fourth and sixth true leaves respectively. Flowering and fruiting of tomato seedlings were also investigated after transplanting. The results showed that the stem diameter and health index of tomato seedlings with cold-shock were enhanced by 7.2% and 55.5% compared with seedlings without cold-shock. Mesophyll cells of the seedlings with cold-shock arranged loosely and various organelles such as chloroplasts and mitochondria were morphologically integrated, while chloroplasts and mitochondria of seedlings mesophyll cells without cold-shock swelled up and thylakoids vacuolized apparently. The flower bud differentiation process of seedlings with cold-shock could be advanced significantly at the early seedling stage compared with the control and the advancement was weakened with the seedling growing. Fruit set number and percentage on the first and second inflorescence of tomato plants transplanted by seedlings with cold-shock were enhanced significantly compared with those of the control. These results indicated that the injury of membrane structure of various organelles, especially chloroplast and mitochondria could be alleviated by cold-shock treatment under high temperature tress. Cold-shock treatment could not only improve the seedling growth under high temperature stress, but also advance the process of early flower bud differentiation and improve the fruit set.