The disease development and some physiobiochemical indices of 3-4-year-old Pinus thunbergii inoculated with Bursaphelenchus xylophilus were studied. The results showed that after nematode invasion, the disease expanded from the base to the upper part of the main stem, there was an interruption of resin exudation, drying and pith browning occurred sequentially in stem xylem, then the needles wilted and finally the plant died. The number of nematodes increased continuously and rapidly, and increased significantly just after symptoms appeared in pine seedlings. The stem water content decreased gradually, stem and leaf relative water content in a new season branch began to decrease not long before symptoms appeared, and the optical density of organic acid extract from the stem of pine seedlings increased. It is suggested that stem water content could be used as a disease indicator, and organic acid would be the most reliable physiobiochemical index for early diagnosis of the disease.