Abstract: To explore the dwarfing mechanism of tomato seedlings induced by coldshock treatment followed by high temperature, tomato seedlings were subjected to coldshock treatment once a day at 8:00 with temperature of 5, 10 and 15 ℃ for 10, 20 and 30 min, respectively, and ethylene production rate was measured. Plant height, ethylene production and gibberellin (GA₃) content of the seedlings treated with T10 ℃D10 min (coldshock with 10 ℃ for 10 min), coupled with utilization of growth regulators, were also evaluated. The results showed that the release of ethylene was increased with the decrease of coldshock temperature and extension of treatment time. The coldshock treatment of 5 ℃ and 30 min had the highest ethylene production rate of 60.3 nL·h^-1·g^-1, which was 6.5 times of the control. None of ethephon (ETH), silver thiosulphate (STS), GA₃ or paclobutrazol (PP333) could completely block high ethylene production induced by coldshock treatment. Tomato seedlings with coldshock treatment (T10 ℃D10 min) resulted in reduction in GA₃ content by 38.1% compared with the value of control (130.6 μg·g^-1). Neither ethephon nor STS had significant effect on the dwarfing induced by coldshock. However, GA₃ weakened the dwarfing effect induced by coldshock treatment (T10 ℃D10 min), while PP333 greatly enhanced it. The dwarfing effect by coldshock treatment of T10 ℃D10 min was equivalent to that of application of 4.0 mg·L^-1 PP333 based on the seedling height as an evaluation indicator. It was concluded that coldshock treatment stimulated shoot ethylene production and blocked GA₃ synthesis. GA₃ played a vital role in dwarfing effect on tomato seedling induced by cold-shock treatment. Coldshock with 10 ℃ and duration of 10 min could promote the growth of tomato seedlings with shorter stem and higher dry mass accumulation.