Abstract: The laboratory and glasshouse experiments were conducted to evaluate the effect of fraction 4 on seed germination and seedling growth of barnyardgrass (Echinochloa crusgalli (L.) Beauv.), eclipta(Eclipta prostrata(L.) L.) and rice(Oryza sativa L.), which was separated from the essential oil of rhizomes of grass-leaved sweetflag(Acorus tatarinowii) that was dephlegmated at low pressure. A common phenomenon was observed in both laboratory and greenhouse trials, that is, seed germination and seedling growth was inhibited at higher concentration but slightly increased at 50 mg/L or lower concentration. Under laboratory conditions, inhibition was observed at concentration from 50～400 mg/L and more than 90% inhibition with >400 mg/L. However, under glasshouse conditions, inhibition occurred at 3000 mg/L or higher while increase of the growth was observed at 1000 mg/L or lower concentration.