Idiopathic thrombocytopenic purpura(ITP) is a hematologic disorder which causes thrombocytopenia. The diagnosis of ITP is based on the history, physical examination and, complete blood count, and examination of the peripheral smear. The diagnostic criteria of ITP established by the Ministry of Health, Welfare, and Labor in Japan requires the bone marrow examination and the measurement of platelet associated IgG, but those tests are not always necessary according to the guidelines developed by the American Society of Hematology. The appropriate strategies for the diagnosis of ITP need to be established. In this paper, some new examinations which may help the diagnosis of ITP are also demonstrated.