Abstract: Effects of 20% isoprothiolane·hymexazol EC (20% IH)on rice seedling grown under natural rained condition were studied using Oryza sative L. varieties Xuan 1 and Yuefu as test materials. After spraying with 300 mg (a.i)/L 20% IH EC solution, the seedling height was inhibited , the leaf sprouting rate accelerated, the root/shoot ratio increased by 46.1% and 10.9%, the roots vitality increased by 15.0% and 109.1% respectively, and the root regenerating ability increased remarkably. The contents of chlorophyll, water-soluble protein and ratio of chlorophyll a/b were also increased. The levels of endogenous hormones in the rice plant were changed with the application of 20% IH EC . The contents of GA4, IAA and CTKs/IAA value in the leaf and sheath were decreased , while the CTKs (including Z+ZR, iP+iPA and DHZ+DHZR) in roots increased. These may be the initial factors that affected the growth and physiological functions of the seedling. After transplanting, seedlings grown from seeds soaked in 20%IH EC had no green-return period, they had higher tiller number and higher resistance to stress condition. Thus it proved that 20%IH EC treatment could improve the quality of rice seedling significantly and was a simple, safe, effective and practical method for raising rice seedlings.