The aim of this study was to examine the possibility effects of betamethasone on a differential white blood cell counts in the hen،s Egg test – chorioallantoic membrane (HET- CAM) test as a novel in vivo model. As well as in this study was manifested this model had to all types of mature white blood cells of chick embryos in age 12-15 days of incubation period. Because of the short period of chick embryos were treated with betamethasone (only 120 minutes) did not showed significantly changes in leukocyte counts (Neutrophils, Lymphocytes, Monocytes, Eosinophils and Basophils) in treated groups with different doses of betamethasone (3, 6, 12, 24, 48, 96, 192 μg / g of total egg weight, topically). We conclude in this study that, the (HET- CAM) test may be used as in vivo model to study the changes in deferential leukocyte counts, because of the blood smear have all types of leukocyte, and the work was appeared in this model have low cost accompanied with other laboratory animals.