|| Avian coccidiosis is most important entero-parasitic disease in the world. Eimeria parasite is causative agent of this disease. In Japan, this parasite species were widely spread and the positive rates are about 50 in layer and 70 % in broiler. The symptoms of coccidiosis are diarrhea, bloody excretion, weight loss, and die. Eimeria tenella is the most pathogenic protozoa. The sporozoites, infection form of this parasite, entry to epithelial cells around the crypt of cecum in early infection. After infection, parasites proliferate in epithelial cells, and form to sexual stage finally. However, we have less information about the pathophysiology, especially invasive mechanisms and infection route, by E. tenella infection. We have focused to analyze the invasive mechanism and route of this parasite because this phenomenon is first event to cause the pathophysiological changes in the infection. I would like to inform about Eimeria parasite and introduce our research in this paper.