The general objective of this study was to investigate and describe the conceptualization of demons along with the process of treatments for mental illness of the Ethiopian Orthodox Tewahdo Church in Addis Ababa, Ethiopia. To meet this objective qualitative research approach, case study research method, was found to be appropriate. Entoto Hammere Noah Kidane Mihret Monastery was selected as a unit of analysis. From this unit of analysis, it was possible to draw sub units namely Exorcist/Priests, Treatments, and Clients. In fetching the required data, in-depth interviews were conducted with religious healers (n=4) and patients (n=2). Direct observation was also employed to study the available treatment processes and reactions of clients during treatments. A theme based frame work was followed to analyze the findings of the study; and the final reporting of the findings followed a linear analytic structure. Based on the posed research questions, eight themes that are grouped under three clusters, i.e. 1. how the story began comprising demons and possession as conceptualized in EOTC, origins of demons, possessing spirits, symptoms of a demoniac, causes of demonic possession, types of possession 2. the healing process in EOTC that include exorcism and healing rituals, and 3. Case presentations were developed. Demonic possession is believed to be the main reason of mental illnesses in the Ethiopian Orthodox church. Demons are recognized to be the archenemy of human race. In their aims and ways of attack, demons vary. There are many evil spirits or demons (including Zar, Buda, Ayine Tila, and Ye digimt Menfes) that are determined to possess human beings and inflict sickness. They possess people starting from their mother’s womb or after they are born. Some evil spirits are catching. They transmit from one host to the other by using parts of the body, an eye for example, as a bridge or through other means like conjuration. Once people get possessed by the evil spirits, they show different common symptoms. Possessions may be of a different type. Pathogenic possessions for example are distinguished from executive possessions based on the way the evil spirits manifest themselves in/through the host. Because the EOTC considers mental illness as the result of demons mischief, different symptoms of mental illnesses are considered to be the tricks in manifestation of the demons. There are different treatments available for the various problems of the clients. The treatments include Holy Water, Holy Oil, Cross, and Holy Ash. The main objective of the treatments is then flashing of driving out the possessing demons from their host.