Abstract: In order to investigate the effects of thiacloprid 360WG on the growth of tomato at doses of 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mg (a.i.) per plant after soil application under greenhouse condition, the tomato seedling's growth, protective enzyme activities in leaves and root vigor were measured 30 days after treatment. The results showed that thiacloprid at 1.0-2.5 mg per plant could increase the leaf numbers, plant height, root length, shoot fresh weight and root fresh weight of tomato seedling. No significant difference was observed between the treatments of 3.0 mg thiacloprid per plant and the control on tomato growth, while the treatment of 5.0 mg thiacloprid per plant showed a slight inhibitory effect. The activities of superoxide dismutase (SOD), peroxidase(POD), catalase(CAT), L-phenylalanin ammo-nialyase(PAL), polyphenoloxidase(PPO) and root vigor were increased for all the thiacloprid treatments. The highest SOD 14.2 U/(min·gFW) and PPO 47.9 U/(min·g FW) activities were observed in tomato seedlings treated with 2.0 mg thiacloprid per plant, which were 49.5% and 29.6% higher than that of the control, respectively. The activities of POD, CAT and PAL reached the maximum after the treatment of 2.5 mg thiacloprid per plant, which were 46.0, 62.4 and 26.0 U/(min ·g FW), respectively and 47.0%, 36.7% and 35.3% higher than that of the control. The root vigor of tomato seedlings treated with 1.0-5.0 mg thiacloprid per plant were enhanced by 2.7%-33.1% compared to that of the control. The results suggested that soil application of thiacloprid at suitable doses could promote the growth of tomato seedlings.