Abstract: This experiment was conducted to study the mechanism of Subgroup J-avian leukosis of Egg-type Chicken Breeder. The disease samples from an eggtype chicken (200-day old) flock in Jining, Shandong Province were studied by histopathological and ultrastructural pathologic methods. The antigen and antibody of avian leukemia were tested by ELISA test kit. The virus was isolated and detected by Indirect immunofluorescence monoclonal antibody test. The sick flocks showed clinical signs of depression, loss of appetite, and extreme emaciation and they finally got paralysed and died. The mortality rate was about 10%. In the necropsy of five dead chickens, almost all livers and spleens showed moderate to severe enlargement with diffuse graywhite nodules; Histopathological examination showed that focal tumor cells, with spherical eosinophilic granules in the cytoplasm, were found in the kidney, spleen, lung, liver. Diffused lymphocytes and lymphoblasts were found in those tissues. At the same time, the virus particles were appeared in the spleen tissues and bursa of Fabricius, about 100 nm in diameter, which are enveloped and spherical. Results showed that p27 antigen and serum antibody were positive in the disease samples, and ALV-J was isolated from them. On this basis, we concluded that the disease in the eggtype chickens was caused by subgroup J Avian Leucosis virus. As there are more reports of ALV affection in commercial layer chickens, special attention should be called.