Abstract: To investigate inhibition activity of Coptidis rhizoma dicoction against Riemerella anatipestifer and its effects on morphology of bacterial body, Coptidis rhizoma dicoction were prepared with water extraction and alcohol sedimentation method. After vacuum distillation, Ethanol extracts of Coptidis rhizoma decoction (EeCrd) which contained crude drug 176 g·L^-1 was acquired. This extracts was bi-diluted to make graded concentrations which contained crude drugs 88,88×2^-1，88×2^-2 ……88×2^-9 g·L^-1, Minimum inhibitory concentration (MIC) and sub-inhibited concentration of EeCrd on Riemeralla anatipestifer in vitro were determined by both disc diffusion test and tube broth assays. Morphology of RA were observed by transmission electron microscope in concentration groups of 88 and 88×2^-3 g·L^-1 of EeCrd in 17 and 3, 6, 17 h, respectively. MIC of EeCrd on RA is 88×2^-2 g·L^-1 concentration group by both two methods, sub-inhibited concentration are 88×2^-3 g·L^-1. Above 88×2^-4 g·L^-1 concentrations, EeCrd almost have no any antibacterial activities. Under 88 g·L^-1 concentrations of EeCrd with 50 μL in 17 h, lengths of RA body concentrated and got shorter, ultimately died. In 88×2^-3 g·L^-1 of EeCrd in 3, 6 , 17 h, lengths of RA body became longer, cell wall got thinner, the fold inside the body reduced, the texture becomes uneven, finally withered, folded, bent and died. EeCrd achieve the purpose of inhibiting the proliferation of RA by destroying the structure of RA cell wall.