The effects of the Smad3- knockout on the hematopoiesis of mouse were investigated in this work. Five pairs of wild type and Smad3- null mice were studied. White blood cell(WBC), red blood cell(RBC) and platelet (PLT) counting of peripheral blood cells were performed with blood obtained from tails. And white blood cells were classified by their morphology. Bone marrow nucleated cells (BMNCs) were counted and classified. The CFU-GM, BFU-E, CFU-GEMM yields were measured in each pair of mice. CFU-S yield of each mouse was measured by injecting bone marrow cells into lethally irradiated 8-10 weeks old wild type female mice. And the pathomorphism of their bone marrows, spleens and livers were observed. As a result, WBC and PLT of Smad3- null mice were significantly higher than those in wild type mice. Smad3- null mice had much more proportion of granulocytes in classification. There wasn't any difference in RBC counting and BFU-E measurement. The yield of CFU-GM increased, while the yields of CFU-GEMM and CFU-S markedly reduced. Bone marrows are actively proliferative, with granulocytosis. The granulocyte/erythrocyte ratio increased. There were no obviously alterative in spleen and liver. Thus Smad3- knockout results in a decreased number of stem and progenitor cells. Moreover hematopoietic differentiation is abnormal with a tendency to forming more granulocytes and platelets. The effect of Smad3 on hematopoiesis is correlative to that of TGF-beta.