Oak gall (Quercus infectoria G. Olivier) has many benefits. It is used as natural astringen material for treating infections such as candidiasis and also as a burn remedy to help heal the skin and prevent infection. The purpose of this research was to determine the characteristic of simplicia, phytochemical screening and the ethanol extract effect of oak gall on cellular immune response of mice. Determination of simplicia characterization was carried out on dry powder oak gall simplicia. Phytochemical screening was performed to simplicia and extract. The extract was macerated with 80% ethanol then evaporated using rotary evaporator to get a thickened extract and dried using freeze dryer. Ethanol extract oak gall was tested for its cellular immune response in mice by counting the total number of white blood cells (leukocytes) and differential count of leukocytes. The results of characteristics’ examination of dried powder simplicia oak gall showed a water content 7.97%, water soluble extract 56.46%, ethanol soluble extract 60.59%, total ash content 1.60% and acid soluble ash content 0%. Phytochemical screening result showed that ethanol extract oak gall positively contains alkaloids, glycosides, flavanoids and tannins. Leukocytes count decreased after cyclophosphamide induction and increased after administration of ethanol extract oak gall at dose of 50, 75 and 100 mg/kgBW. The differential leukocyte count obtained the neutrophil cells, lymphocytes and monocytes. Ethanol extract oak gall (quercus infectoria g. olivier) has an effect on the cellular immune response of mice and it showed a dose-dependent manner.