Abstract: Pathogenesis were studied by inoculating the broiler chickens with reticuloendotheliosis virus(REV) and Marek’s disease virus (MDV) . After 13 days post-inoculation, broiler chickens began to die and the mortality reached 84% at the end of experiment. By gross examination, there were two different tumors of sporadic, large tumors and diffusible, small tumors that induced and mixed on the aspect of liver, spleen, kidney and heart. These samples were embedded with paraffin and cut into a series of segments that were stained with hematoxylin and eosin, and examined for existence of two different kinds of tumor cells by microscopic lesions. Some segments were reacted with monoclonal antibody of MDV BA4 and REV 11B118 respectively. The IFA examinations showed that both MDV and REV antibody could react with the same segment, which meant REV and MDV could induce tumors separately in chickens.14,21, 28 and 42 days after inoculated, both MDV and REV were isolated from three blood samples that were collected at random. The results showed that coinfection of MDV and REV could prolong the date of virus existence in the blood.