Histomoniasis is a new disease in Vietnam, which is major cause of economic loss in poultry farming in Phu Binh, Thai Nguyen. Observation clinical 120 infected chickens 3 - 4 months, the necropsy in 52 chickens, after that choose 20 chickens have characteristic lesions to fix in formalin-fixed, paraffin-embedded tissues, Hematoxilin - Eosin staining, use optical microscopy to observe. The results showed that: Clinical signs include: The first signs seen in an infected flock may just be anorexia, depression, listlessness, drooping wings, unkempt feathers and sulphur-yellow diarrhoea. Chickens may be sick for some time and become emaciated before death. The primary gross lesions in cecum and liver. Cecum exhibit marked inflammatory changes and ulceratons, causing a thickening of the cecal wall. The cecal pouches contain a yellowish green, caseous exudate or, in later stages, "a dry, cheesy core". Liver lesions are highly variable in appearance, enlarged liver spotted with target-like lesions representing areas of necrosis, or cell death with gray yellow color. Microscopic lesions is typical cecal lesions with the lining of ceca is destroyed, lost completely the structure of the intestinal epithelium. Instead of pink necrosis and filled the typical oval follicles. In the liver, liver cells is necrosis large masses, pink colour. In necrosis areas, filling with the oval follicles, egg-shaped follicles. In other areas, liver cells are degeneration and infiltration of many inflammatory cells as heterophile and eosinophile.