Abstract: Citrus Huanglungbin Organism is pleomorphis.The diameter of its spherical body is 50-500ntn, and the size of its rod body is 40-70x200-2500nm.It has an envelope, with 25nm in thickness, which comprises three layers of unit membrances.The outer electron-dense, the inner electron-dence and the middle electron-clear layers are, respectively, similar to cell wall with 7-10nm in thickness, protophasmic membrane with 3-8nm in thickness, and periplasmic space with 7-15nm in thickness.Both the outer cell wall and the inner protophasmic membrane are composed of two distinguishable membranes.After the treatment by papain, an electrondense layer with 5-8nm in thickness was observed under electron-microscope, which is located in electron-clear area between the outer cell wall and inner membrance, and similar to a piplidogly-cane layer between the ceil wall and the membrance of Escherichia coli.According to the presence of piptido-glycane layer in the cell wall, citrus Huanglungbin organism may be Gram-negative bacterioum, there is a relatilve linkage between the quantity of pathogen in the sieve cell of citrus cultivars and their susceptibility.