Because of its strong resistance to environmental stresses, Alternanthera philoxeroides, an alien invasive plant, has fast become a severe weed in China. In order to explore the mechanism of its strong resistance and invasiveness, we studied the properties of its chitinase, one of the pathogenesis-related proteins in plant. The protein was extracted from the leaves treated by ethylene, precipitated with 40～60% ammonium sulfate and isolated by Sephadex G-75 chromatography. An enzymatic assay showed that the optimum pH for this enzyme was about 5.7 and it was stable in the range of 4.0 to 8.0 pH. It would loss more than 50% of its activity when pH was lower than 4.0 or higher than 8.0 and almost had no activity when pH was 2.0 or 10.0. It had high endurance to wide range of temperatures and its activity was high from 30℃ to 80℃. Its optimum temperature was about 70℃ and it began to loss its activity when temperature was higher than 80℃. It was also found that the reaction velocity of this enzyme increased with the increase of its substrate concentration when the concentration was lower than 1.25%. All of these suggested that high endurance of this enzyme to temperature and acidity or alkalinity might have contributed to Alternanthera philoxeroid's strong resistance to environmental stresses.