Abstract: Based on the strong motion data of the 1999 M_W7.6 Chi-Chi （Jiji） earthquake in Taiwan region, the relative motion time histories of sites across the fault are studied. In order to obtain accurate residual displacement of near-field ground motion, a base-line correction method using variable step-size grid search is proposed. Through the correction of the initial time of the ground motion history, relative motion histories between the hanging-wall and footwall stations of the Chi-Chi earthquake are obtained. The results show that the fling-step effect of the ground motion is significant, the average slip velocity of the fault plan can be as high as 1.4 m/s; the relative motion between the fault plans is similar to the motion characteristics of the fault ground motions, while the amplitudes are even larger; the residual displacements of the fault are about 80% of the maximum inter-plan relative displacement.