Abstract: The reaction of pine seedlings to infection of pine wood nematode(Bursaphelenchus xylophilus) was similar to that of in vitro seedling twigs. Both showed the sign of wilting and tissue brown spot, the reaction of 2 or 4-month seedlings to PWN appeared clearer and faster than that of 1-year-old pine seedlings and in vitro seedling twigs. After being infected by PWN,the needle of 1-year-old pine seedlings with the lignified stem began to change to be bronzing or brown and wilted, which appeared at the phyllopodium and expanded to leaf apex, then the needles withered and died. The sign showed that the disease had reached an advanced stage,where the host was almost dead and water loss in the stem and cell necrosis had widely occurred. The experiment showed the same results in in vitro seedlings. The necrosis was a common phenomenon caused by PWN and directly related to its moving, spreading, distribution and amount. The visible brown spot at cortex and marrow appeared after nematode occurrence, and the degree of internal pathological changes was linear to nematode amount.