Hemocytes are the cells that have a part in the insect's cellular and humoral immune systems. Yet, characterization of cells depends on light microscopic studies. In addition, using different names for cells generally may cause confusion. The present study aimed to characterize the morphological types of hemocytes in the oriental cockroach Blatta orientalis (Blattodea: Blattidae) (potential vectors of human allergies and asthma) using transmission electron microscopy. The study was conducted at the EGEMIKAL Analysis and Histology Laboratory of Ege University during 2019-2021. The circulating hemocytes in the ten individuals' adults (5-day old) of B. orientalis were characterized as four types based on their morphology. These were prohemocytes, plasmatocytes, granulocytes and oenocytes. The first three types of hemocytes were easily identified by their morphological features compared with other insect hemocyte types. This is the first report on the identification of oenocyte type cell in B. orientalis. The cell appeared large and elliptic. The nucleus was elongated, bi-nucleate and slightly opaque in cell. The cytoplasm contained several vacuoles filled with electron-dense materials. More comprehensive knowledge of hemocytes may help us understand better the cockroach cellular defense system.