Changes of benzoic acid,phenylacetic acid and salicylic acid contents in stem of 3-4 year old Pinus thunbergii inoculated with Bursaphelenchus xylophilus were studied by HPLC.The results showed that after nematode infection,contents of benzoic acid and salicylic acid in the stem of Pinus thunbergii increased gradually and then decreased when serious disease occurred,while content of phenylacetic acid increased at early stage and decreased at later stage of the disease.The role of benzoic acid,phenylacetic acid and salicylic acid during the development of the disease was discussed. Changes of benzoic acid, phenylacetic acid and salicylic acid contents in stem of 3-4 year old Pinus thunbergii inoculated with Bursaphelenchus xylophilus were studied by HPLC. The results showed that after nematode infection, contents of benzoic acid and salicylic acid in the stem of Pinus thunbergii increased gradually and then decreased when serious disease occurred, while content of phenylacetic acid increased at early stage and decreased at later stage of the disease. role of benzoic acid, phenylacetic acid and salicylic acid during the development of the disease was discussed.