Transmitting test on brown bast of rubber trees by bark-grafting gave following results: 1) Three months after grafting, typical brown bast symptoms displayed on the trunks of the originally health stocks. The incidence of disease was 29-50;. The symptoms were water-soaked, brown lesion on the bark, dryness on the cut and less latex. 2) Assaying by means of micro-precipitation and ELISA showed that trunks with brown bast accounted for 40-45;of the originally healthy stocks 13-14 months after grafting. 3) Examined under electronic microscope, Phytoplasma were discovered in the cells of the phloem tissues from the trunks of the originally healthy stocks. The size of the Phytoplasma was 300-850mm, with one layer of membrane in 9-12 mm thickness. No these Phytoplasma were discovered in the cell of the phloem tissues from the controlled trunks (healthy stocks, not grafting with diseased scions).