Abstract: In the interaction between Pinus thunbergii and Bursaphelenchus xylophilus, nitric oxide (NO) is an important signaling molecule involving in the early response of P. thunbergii to the invasion of B. xylophilus. However, it is unclear that whether the NO production by P. thunbergii is triggered by the invaded B. xylophilus or its secreted metabolites. In the present study, the P. thunbergii was inoculated with living B. xylophilus, its secretion, and the suspension of grinded B. xylophilus, respectively, and the nitric oxide synthase (NOS) activity and NO content in the P. thunbergii were detected at the early stage. In all treatments, the inoculated P. thunbergii appeared disease symptoms, and the NOS activity and NO content in the P. thunbergii inoculated with B. xylophilus secretion and grinded B. xylophilus suspension increased, suggesting that besides living B. xylophilus, its contents or secretion could also trigger the expression of NO response signal in P. thunbergii, inducing the downstream response and causing the disease development of P. thunbergi. With the increasing temperature at 15-25 ℃, both the NOS activity and the NO content in inoculated P. thunbergii increased, and the disease symptoms appeared earlier. The same patterns of NOS activity, NO content, and disease symptoms were also observed under increasing drought stress. It was suggested that within a definite range, increased temperature and drought stress could enhance the NO signal expression in inoculated P. thunbergii and accelerate its disease development, and thus, the disease development of inoculated P. thunbergii under high temperature and drought condition could be related to the enhancement of the NO response signal in the host.