Abstract: In order to test the bioactivity of ethanol extract from Eichhornia crassipe, we tested 3rd instars and last-instar larvae of Tenebrio molitor in different concentrations by using topical application method. The results showed that the ethanol extract was strongly poisonous to the 3rd instars, and the mortality significantly increased in all groups which were treated with different concentrations of ethanol extract comparing with the control group, the mortality could reach to 73.33% when concentration was 50 mg/mL. However, the weight gaining and moulting time of 3rd instars were significantly decreased and postponed respectively only when the concentration reached to 50 mg/mL. Additionally, the mortality and malformation rate of the last -instar larvae increased and the emergence rate decreased decreased with the ethanol extract concentration increasing.