Abstract: The effects of p-hydroxybenzoic acid (p-HA) and cinnamic acid (CA) on the growth and the antioxidant enzyme activities in leaves of Eucalyptus grandis×E. urophylla No.9 were analyzed. The research material are seedlings in March of E. grandis×E. urophylla No.9 in substrate culture. The results showed that the growth of seedling height and ground diameter in the 4 concentrations of p-HA (10, 50, 100 mg/L and 500 mg/L) were significantly higher than CK, especially the seedling height increment was up to 52.33 cm under the p-HA10 mg/L. The seedling height increment under CA 10 mg/L was higher than CK, and the seedling height increment and the ground diameter increment gradually decreased with the increase of the treatment concentration. Compared to CK, the total chlorophyll content in the leaves was significantly lower and the malondialdehyde (MDA) content was slightly higher. Both p-HA and CA could change the activity of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD) and polyphenoloxidase (PPO). The synthetical allelopathy index of p-HA 10, 50 mg/L and 100 mg/L and CA 10 mg/L was positive, and this index of p-HA 500 mg/L and CA 50, 100 mg/L and 500 mg/L was negative. It can be seen that both p-HA and CA can promote the growth of E. grandis×E. urophylla No.9 seedlings at low concentration and inhibit it at high concentration. The concentration of inflexion point are p-HA 100 mg/L and CA 10 mg/L. The allelopathy of CA was stronger than that p-HA at the same concentration.