Effects of different concentration of chlorpyrifos on antioxidant enzyme(SOD, POD, CAT) activities, malondiadehyde(MDA), proline(Pro), soluble sugar, and deoxidization Vc contents in pakchoi（Brassica campestris L. ssp. Chinensis Makino var. communis Tsen et Lee）seedlings were investigated under greenhouse conditions. The results showed that the antioxidant enzyme activities were significantly influenced with both low and high concentration of chlorpyrifos spayed on the pakchoi seedlings. Most of these antioxidant enzyme activities showed to decline firstly and then to rise. Five days after treatment, the chlorpyrifos stress significantly increased the activities of SOD, POD, CAT in pakchoi, comparing with the control. Among of them SOD′s activity showed the highest level. Until 21d after the treatments, the activities of SOD, POD, CAT reinstated to the contol level. The MDA content was increased under chlorpyrifos stress, especially when spraying high concentration of chlorpyrifos, it was suggested that chlorpyrifos stress promote the lipid peroxidation. And the content of Pro was also increased in 3d,5d after the treatments. On the 7th d, Pro content reached to the control level, while. The contents of soluble sugar and deoxidization Vc were reduced in the early stage of the stress, and then rised to the control level. It was concluded that a long time was needed for the plants to recover from the pesticide stress.