Dengue fever is the common infectious disease in the tropical area and in Viet Nam. Study on the character of blood cells and the relation between thrombocytopenia and hemmorhage in 124 Dengue fever's patient at Dong Anh hospital from september to november 2009, there were: disease was in both male and female, man/woman:0.9, average age:31, the commonest problem is fever. There are clearly changes in the number of blood cells, especially is low platelet and white blood cells count. The 6th febrile day before admission has lowest count. Bleeding under the skin is the commonest hemorrhage symptom, but the more platelet deficiency the more bleeding under the skin increases.