Abstract: A field experiment was conducted to investigate effects of four different applied doses of 5.0% imazethapyr on growth of alfalfa (Medicago sativa) seedlings and root nodules in Heilongjiang. The results indicated that application of imazethapyr at different levels (0.067 5、0.082 5、0.097 5、0.112 5 kg/ha) leaded to the different negative effect on alfalfa emerging and seedling growth. The dosage at 0.112 5 kg/ha had stronger inhibitory effect than those of others, while the effect of 0.067 5 kg/ha was the least. The study also showed that the negative effect on increase of nodule number and their dry weight accumulation with high dosage of imazethapyr lasted longer, while medium and low dosages had shorter inhibiting time and easy resuming from inhibition. There were significant differences (P0.05) of dry weight of alfalfa seedlings at the different dosages applied after 10 weeks, but the difference of total nitrogen contents was not significant. Therefore, the dose of imazethapyr at 0.067 5 kg/ha might be a safety level on growth of alfalfa seedlings.