Abstract: The effect of superoxide anion( O2-) at the initial time of pine wilt disease was studied in this paper. Japanese black pine seedlings (Pinus thunbergii) of 2 years old were inoculated by three different kinds of virulent strains pine wood nematodes (PWN, high virulent and mild virulent strain of Bursaphelenchus xylophilus and avirulent strain of B. raucronatus). The stem and leaf of samples were taken to determine the O2- content and superoxide dismutase (SOD) activity in P. thunbergii at 4,12,24,48,72,96 and 120 h after inoculation respectively. It showed that O2- and SOD in P. thunbergii which were inoculated with three different kinds of virulent strains PWN displayed distinct different changes. All treated seedings brought forth the first O2- burst in 12 h. The reverse correlation between O2- content and virulent strain could be founded in the pine stem. The second stage of O2- increase in leaf of the seeding inoculated with avirulent strain of B. mucronatus was earlier than that inoculated with high virulent B. xylophilus. There was a biphasic SOD burst in the pine after it inoculated with avirulent strain of B. mucronatus, while the pine inoculated with high virulent PWN only had single SOD burst. After the first increasing stage of Mn -SOD(24 h) ,the Mn -SOD content in seeding inoculated B. mucronatus was higher than that inoculated with high virulent PWN. The rising of O2- content was coincident with SOD. Both of them had same integrative trend. However, SOD had slower change than O2- . So it was indicated that O2- was an important juncture in pine wilt disease resistance signaling pathways.