To observe the morphological characteristics and hematopoietic function of bone marrow megakaryocyte (MK) in children patients with idiopathic thrombocytopenic purpura (ITP), and to preliminary analyse the cause and mechanism of thrombocytopenia. CD41 McAb immunohistochemical technique was used to detect micromegakaryocyte in bone marrow smear. Plasma clot culture and CD41 McAb immunohistochemical technique were used for the MK-colony forming assay. The results showed that there was no statistical difference of the positive rate of micromegakaryocyte between groups of ITP and control, but type I lymphocyte-like micromegakaryocyte was infrequent. The number of micromegakaryocyte and the formation rates of CFU-MK and BFU-MK in ITP group were significantly higher than those in control group. The normal MK releasing platelet could be easily found in the culture system. The MK colony formation rate was decreased in a patient with chronic ITP. In conclusion, the increment of type II, III, IV micromegakaryocytes is one of pathologic phenomenon of ITP. These small megakaryocytes can develop and mature to normal megakaryocytes in the condition of ex vivo culture. The developmental abnormity of MK is a possible reason for thrombocytopenia among partial patients with ITP, especially the chronic cases.