Mikania micrantha is a dangerous exotic weed that is now widely distributed in Guangdong, china. The lepidopteran defoliator Actinote thalia pyrrha (Fabricius), introduced from the Indonesian Oil Palm Institute, is a potential biological control factor for M. micrantha. The change in activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), polyphenoldoxidase (PPO), the content of total phenolics and the capacity of scavenging 1,1-diphenyl-2-picrylhydazyl (DPPH.) free radical in M. micrantha leaves were analyzed after infestation by the larvae of A. thalia pyrrha. The results showed that a slight change of all the indexes in damaged leaves appeared within 3h. After four days of A. thalia pyrrha infestation, the activities of SOD and POD in damaged leaves were higher than the control, but the activity of CAT was lower than the control indicating that the functions of SOD and POD were impeded more than CAT. The activities of SOD and POD reached their hightest value after 48h of A. thalia pyrrha feeding, then decreased, while the highest activity of CAT occurred after 24h before decreasing. The activity of PPO fluctuated greatly, its three peak values were 1.83, 1.92 and 2.17 times that of the control. Total phenolics content increased at first then decreased after 72h of A. thalia pyrrha feeding. The capacity of scavenging DPPH. was significantly lower than that of the control (p＜0.05). SOD activity was positivity correlated with CAT and POD activities for both damaged and normal leaves, however the value of correlation coefficiency in damaged leaves was higher than in normal leaves. The content of total phenolics also showed a weak positive correlation with PPO activity. It is suggested that the protective response of M. micrantha to A. thalia pyrrha infestation was short and limited, but resulted in reduced function of leaves, disturbed metabolism in the protective enzyme system, and decreased antioxidative capacity. M. micrantha was not able to resist the feeding stress by altering content of total phenolics.