This study presents a segment-summation scheme for calculating co-seismic deformations caused by a seismic model with spatial distribution of fault slip. The basic idea is to divide such a fault plane into limited sub-faults, so that the co-seismic deformations caused by each sub-fault can be evaluated by applying Okada's formulation <Okada, 1985> and summing up the individual contributions from the whole sub-faults. Two case studies of the Chi-Chi earthquake (Mw7.6, 1999) and the Kunlun earthquake (Mw7.8, 2001) show that there exists a big discrepancy between the results calculated for the two dislocation models. It implies that the mean dislocation model remarkably affect the calculating results. The results of RMS errors show that the co-seismic displacements calculated by the fault with spatial distribution of fault slip improve the results by over 50% compared to the mean dislocation results.