Changes in the Earth's gravity field are caused by the redistribution of mass within the Earth and on or above its surface. While previous studies <Tapley, 2004; Wahr, 2004> showed that the Gravity Recovery and Climate Experiment (GRACE) satellite mission, executed by NASA, had successfully determined oceanic mass redistribution, the relative motions between two GRACE satellites caused by the 2004 Sumatra tsunami is still uncertain. This present study combines a numerical model of the tsunami and GRACE orbit data to estimate the realistic effect of oceanic mass redistribution on the inter-satellite range-rate change between two GRACE satellites. The GRACE mission is designed to map out the Earth's gravity field to high accuracy. Instead of measuring the Earth gravity field or mass variations directly, GRACE estimates a set of spherical harmonic coefficients denoting the Earth gravity field each month by measuring range changes between two spacecrafts <Chambers et al., 2004>