The most serious problem resulting from estrogen deficiency induce osteo porosis. Recently, they make efforts to inquire a relation between hematopoietic organ and bone loss due to estrogen deficiency. Estrogen have an effect on growth and formation of skeletal system, and inhibit bone resorption under the influence of osteoblast and osteoclast, and basically inhibit the increase of hematopoietic progenitor and immune factor connected with bone resorption and prevent the osteoid formation. The purpose of this article was to observe the change of spleen and effect on hematopoietic function following estrogen administration. In this study, female rats of 150g weight was ovariectomized, after 70 days, experimental group was injected estrogen at interval of a week and sacrificed on 1, 2, 3, 4, 6 weeks. Control group was sacrificed after ovariectomy on 11, 12, 13, 14, 16 weeks without estrogen injection, and normal rats were sacrificed for harvest of spleen and femur. Paraffin sections and H&E stain was performed, and observed under light microscope. The obtained results were as follows. 1. From 11 to 12 weeks at bone marrow of control group, hematopoietic cells were decreased in comparison with normal group, and lipid infiltration was seen, and irregular bone remodelling was seen after 13 weeks. From 14 to 16 weeks, there were more decreased hematopoietic cells and lipid degeneration, and lipid degeneration of hematopoietic cells appeared. 2. All the bone marrow of experimental group, the structure of hematopoietic cells with decreased lipid infiltration was recovered from 2 weeks of estrogen adminstration and maintained to 6 weeks. 3. At spleen of control group, borders of white and red pulp was not well demarcated, and size of white pulp was decreased. 4. At spleen of experimental group, borders between white and red pulp have been well demarcated from 3 weeks of estrogen adminstration relatively, and white pulp was increased with distinct border. From above findings, we could regarded that estrogen deficiency due to ovariectomy influenced on hematopoietic cells of bone marrow and spleen, and histologic recovery of hematopoietic cells were observed after 3 weeks of estrogen adminstration even if it was not reach to normal group.