The World Health Organization (WHO) classification of leukemia by morphology, cytochemistry, immunology, genetic and clinical features. The first factor in classifying leukemia is whether most of abnormal cells are mature or immature, can be classified into acute leukemia and chronic leukemia. In acute leukemia, the cells are very abnormal in proliferation and differentiation, so the number of blast cells increases rapidly. Acute leukemia can occur at any age and the disease development is rapid. In chronic leukemia, abnormal white blood cells grows slowly and usually occur in adults and the disease development tends to be slower than acute leukemia. In adults any type of leukemia can be found, whereas most leukemia that found in children is an acute type. The second factor in classifying leukemia is the type of bone marrow cells that are affected. It can be classified into lymphocytic leukemia and myeloid leukemia, lymphocytic leukemia are immature forms of lymphoid cell (e.g.B-cell, T-cell and NK-cell), myeloid leukemia (also known as myelocytic, myelogenous, or non-lymphocytic leukemias) are early forms of myeloid cells (e.g.. red blood cells , platelet and monocytes. The combination of these two factors, leukemia can be classified into four major types of leukemia, including acute myelogenous leukemia (AML), chronic myelogenous leukemia (CML), acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL) (James W, et al.2009).