Giant cell tumors (GCT) of bone are generally benign; but can be locally aggressive. Campanacci et al proposed grading system based on radiographic appearance of tumor. Grade 1 (latent): Well defined margin and intact cortex; Grade 2 (active): Relatively well-defined margin but no radio-opaque rim. Cortex is thinned, moderate expansion of bone; Grade 3: Lesions are aggressive, have indistinct border, and have cortical destruction. Ultrasound with Doppler is useful to evaluate aggressiveness of the tumor by calculating the resistive index (RI) of the lesion. There were 28 cases of extremity GCT. Maximum cases were grade III (n=16), followed by grade II (n=11). Because of cortical thinning or destruction USG was useful in assessing size, site, echo-texture, soft tissue component and vascularity. Most of the tumors are mixed echoic predominantly isochoric solid and cystic component, 5 of 28 cases had ABC component. In our observation 10 out of 11 cases of grade II tumors showed low RI. The 4 cases of grade III tumors having low RI (< 0.64), one of these have thick cortex which may not depict correct RI value. Two of the 14 cases with high RI value had low grades (I or II) and 12 (86% cases) were in grade III. The mean RI of grade III tumors was higher than that of grade II GCT. Thus, USG and Doppler can be used for detecting aggressiveness of GCT.