In this paper, the pathology of "Appendage Shaking Disease" is studied by comparing normal and affected crabs via ultratructural investigation.A microoganism, whose size is between virus and bacterium, measured 0.22-0.35μm, is granular or clavate, bounded by a cell wall and a membrane, but it has nonucleus.In early stage of infection, the microorganisms are mainly located in blood cells, and in the last stage, most ROLs are distributed in the connective tissue of nervous organization, gill cavity, heart, periopod muscle, and digestive system.According to their size;shape, inner structure, locating places, and propagation mode, it can be inferred that they are Rickett sia-like organisms (RLOs)which should be the pathogens of "Appendage Shaking Disease".The pathogeny of RLOs is analyzed and discussed in this paper as well.