Abstract: The paper was to study and compare herbicidal bioactivities of the three eucalypt leaf extracts, so as to lay the foundation for research and development of herbicides, and comprehensive development and utilization of eucalypt. Three eucalypt leaf extracts were bioassayed by seed germination method with sorghum (Sorghum vulgare), wheat (Triticum aestivum), cucumber (Cucumis sativus) and rape (Brassica campestris) as indicators, and the affection of the length of the roots and the stems were studied. All the three extracts showed strong inhibition effect to the roots and the stems of the four indicators, and the IC₅₀ were 0.5076, 0.5540, 0.6493 mg/mL against the roots, and 0.5187, 0.5811, 0.6882 mg/mL against the stems of sorghum; and 0.5117, 0.6717, 0.9062 mg/mL against the roots，and 0.4448, 0.4740, 1.5659 mg/mL against the stems of wheat; and 1.7126, 2.3397, 1.8512 mg/mL against the roots，and 3.0538, 2.0922, 4.0444 mg/mL against the stems of cucumber; and 0.0258, 0.8069, 0.9425 mg/mL against the roots，and 0.2462, 1.3375, 1.4521 mg/mL against the stems of rape. All the three extracts showed high herbicidal activity, among which the extracts of Eucalyptus robusta showed the highest herbicidal activity, then followed Eucalyptus globulus, and Eucalyptus maideni the lowest. So if eucalypt was used as resource to develop herbicides, Eucalyptus robusta should be planted.