Abstract: The experiment adopted peel-grafting method to inoculate the 2 a Pinus thunbergii seedling with strong and weak pathogenicity Bursaphelenchus xylophilus as well as non-pathogenicity Bursaphelenchus mucronatus. The observation results showed that the strong pathogenicity Bursaphelenchus xylophilus spreaded more quickly inside the Pinus thunbergii. The needle begins to turn yellow in 4d. The new tip of pine seedling begin to wilt in 7d, and died completely in 10d. The amount of the nematode was much more than that of inoculated because the nematode reproduced in a great deal inside of the seedling. The weak pathogenicity Bursaphelenchus xylophilus spreaded relatively slowly inside the Pinus thunbergii. The needle turned yellow in 7d. The new tip begins to wilt in 15d. The seedling died in 25d. The amount of nematodes began to increase. After non-pathogenicity Bursaphelenchus mucronatus had been inoculated, only small amount of nematodes from the tree top was separated in 7d. Some needles on the tip turned yellow. The nematode didn’t reproduce basically. The nematode can not be separated from the seedling in 25d. The needle symptom on the tip hasn’t strengthened which doesn’t lead to the death of the new seedlings. Therefore, after the Pinus thunbergii was infected with nematode, whether it wound come down with disease was closely related to the nematode survival inside the pine seedling. The symptom of the disease has direct relations to the nematode spread speed inside the pine seedling as well as whether it can reproduce considerably.