Abstract: In order to determine early reactions of A. thaliana to polycyclic aromatic hydrocarbon phenanthrene stress, early responses of antioxidases and membrane protection system of A. thaliana were analyzed. Then differential expressions of related genes in mRNA level under phenanthrene stress were determined via Real-Time PCR. The results show higher superoxide dismutase (SOD) and peroxidase (POD) activities compared with non-treated control samples when A. thaliana seedlings are exposed to phenanthrene for 12 h. This may indicate that the plants are activated inand H2O2. Excess H2O2 begins to precipitate when A. thaliana is exposed to phenanthrene for 72 h, but the membrane system shows no obvious injury from lipid peroxidation. While phenanthrene stress prolongs, A. thaliana photosynthesis is completely inhibited, which is indicated by at gene transcriptional level and/or at cellular level. The findings suggest that oxidative stress reaction is an important early response to polycyclic aromatic hydrocarbons (PAHs) stress in plants.