Two cases of giant cell tumor of bone diagnosed by fine needle aspiration cytology are described. Case 1 was a 28-year-old male who had pain sense for one year at the right distal thigh. His radiologic finding revealed a destructive cortical lesion with soft tissue extension at medial side of epiphysis of the distal femur. Case 2 was a 21-year-old female complaining pain at left distal forearm for eight months and showed a well-demarcated expansile osteolytic lesion with multiseptation, and cortical destruction at epiphysis and metaphysis of the left distal radius on the X-ray. Fine needle aspiration of each lesion was performed. The aspirate of the case 1 revealed moderate cellularity, which was composed of scattered giant cells of osteoclastic type and small round to oval monotonous stromal cells in large areas. Giant cells were evenly distributed in single or small groups and had irregular but abundant cytoplasms with 10 to 20 nuclei in the center. The nuclei showed ovoid shape, fine granular chromatin, and a small but conspicuous nucleolus. Stromal cells were dispersed in isolated pattern or sometimes aggregated in clusters and showed the same nuclei as those of giant cells and scanty cytoplasms. Comparing to case 1, case 2 had a more translucent abundant cytoplasm in the giant cells and more spindled stromal cells. All two cases revealed neither nuclear atypism nor increased abnormal mitoses in both giant and stromal cells, suggesting no evidence of malignancy. Thereafter the lesions were treated with excision and curettage, and histologically confirmed as giant cell tumors of the bone.