Abstract: With the observed GPS coseismic displacements of Tokachi-Oki Earthquake and its biggest aftershock,we derive the coseismic slip distributions of the two earthquakes using SBIF(sensitivity-based iterative fitting)method.A more real fault geometry and layered Earth model were considered in the inverted process.As results shown,the maximal slips of the main shock and the biggest aftershock are 3.07 m and 0.33 m respectively.The corresponding moment magnitude scale are Mw 7.94 and Mw 7.1,which is consistent with the seismic result.In addition,the slip of the aftershock is mainly located at the place the coseismic slip of the main shock is lower,which perhaps indicate that the stress of surrounding region is enlarged by the coseismic slip of the main shock and thus trigger the biggest aftershock.