It is important to evaluate the biological characteristics of a tumor for better treatment. Generally, histological analysis and imaging, technigues such as roentgenogram, computed tomography and magnetz resonance imaging, were used for this purpose. Also, several researches by using of surface markers of tumor cells and immunologic and biochemical studies were done. But, despite of these efforts, it is still a difficult problem to define the malignancy of a tumor for appropriate treatment. For recent 10 years, flow cytometry, an analysis of tumor cell DNA content and cell cycle, has been introduced and used to evaluate the biological aspect of a tumor, especially the degree of malignancy. In solid tumors and hematologic malignancies, its usage was proved clinically to be effective in deciding the treatment method and predicting the prognosis of the tumor but its efficacy in bone tumor is still under discussion. We had flow cytometric analyses of 11 cases of giant cell tumor (GCT) that pathologically is benign but frequently has malignant course in clinical practice, by using of paraffin-embedded tumor tissues, retrospectively. There was difference in DNA content (ploidy) and S-phase fraction between conventional GCT and Fibrous Histiocytoma-like variant GCT and malignant GCT. But no significant relationship was observed between clinical course (local recurrence and distant metastasis) and DNA content and S-phase fraction. In cases with local recurrence, it showed decrease of number of diploidy in DNA content and increase of S-phase fraction.