Abstract: The Mar 11th 2011 Tohoku earthquake(Mw=9.0) is located in northeastern of Japan.The surface observation data were rapidly obtained using the dense continuously GPS stations covered the island after the earthquake broking.In this paper,the reliable co-seismic deformation field is derived from GPS measurements using the highly precise data processing method and sea-floor points set near the epicenter.The co-seismic fault slip distributions are further inverted to explore the seismic mechanism using the steepest descent method to get the optimal solution.The second-order Laplace smoothing operator method had been used to constrain the fault slip amplitude based on the layered elastic half-space homogeneous model.The inversion results show that the energy released by the earthquake is about 4.48×1022 N·m equivalent to moment magnitude Mw 9.07.The average fault slip is about 6.05m while the maximum slip is about 58.7 m at 143.17 degrees east longitude and 38.25 degrees north latitude according the GPS and sea-floor points observed co-seismic deformation.The seismic fault characteristic is mostly thrust slip slide at the epicenter while some strike slip slide at the edge of faults.