In this study, a method for simulating co-seismic gravity gradient tensor of great earthquakes has been developed. We compare the results with the existing softwares. Moreover, gravity gradient changes of the 11 March, 2011, Tohoku earthquake has been calculated using GRACE data. The GRACE-derived gravity gradient changes as well as gravity change agee well with seismic model prediction. We use a set of the gravitational gradient tensor components (e.g. ΔVxx and ΔVxz, x and z refer to north and up directions, respectively) since they are more sensitive to small-scale signals and they reduce striping errors without using any de-striping filtering.