Diffusion weighted MRI (DWI) is promising for early prediction of tumor response to radiation therapy. We report our results of using longitudinal DWI approach performed on ViewRay system for predicting the response of sarcoma patient to pre-op RT. Six sarcoma patients were recruited in this study. Each patient subsequently underwent surgery. The tumor necrosis score was then used to compare to the ADC values to assess the predictive value of longitudinal DWI. Each patient successfully underwent 3–5 diffusion MRI scans. Based on the data from 6 patients, our longitudinal changes in tumor ADC matched well with pathology necrosis results.