According to the electron microscopic observation the wheat yellow curl-leaf disease wascaused by a rickettsia-like organism (RLO). The morphology and structure of the pathogenand its distribution within the host plant were of typical characteristics of the RLO. RLO_Swere seen in the phloem and xylem of the host plant. In xylem elements RLO particleswere closely related to electron-dense materials spreading in them. Some channels wereformed in the cell walls of protoxylem, and sometimes in the cell walls of mature xylem ele-ments which were eroded. RLOs entered adjacent cells and intercellular spaces via thesechannels. A large number of RLOs were seen in sieve elements and they could get intoadjacent sieve elements via sieve pores. The places of intercellular movement of RLOs weremostly in the plasmodesmata-dense areas or pit fields where the primary cell walls amongplasmodesmata were dissolved to form some channels by the invasion of RLOs. And theforming of less intercellular channels did not relate to plasmodesmata and pit fields. Basedon these results the mechanism of systematical infection of host plant caused by the RLO isdiscussed in this paper. According to the electron microscopic observation the wheat yellow curl-leaf disease wascaused by a rickettsia-like organism (RLO). The morphology and structure of the pathogen and its distribution within the host plant were of typical characteristics of the RLO. RLO_Swere seen in the phloem and xylem of the host plant. In xylem elements RLO particleswere closely related to electron-dense materials spreading in them. Some channels wereformed in the cell walls of protoxylem, and and in the cell walls of mature xylem ele-ments which were eroded. A large number of RLOs were seen in sieve elements and they could get intoadjacent sieve elements via sieve pores. The places of intercellular movement of RLOs were mostly in the plasmodesmata-dense areas or pit fields where the primary cell walls amongplasmodesmata were dissolved to form some channels by the invasion of RLOs. And theforming of less intercellular channels did not relate to plasmodesmata and pit fields. Basedon these results the mechanism of systematical infection of host plant caused by the RLO isdiscussed in this paper.