the effects of soil layers on strong motion duraiton were analyzed by using the strong motion records from 17 earthquakes recorded by the SMART1 array. The strong motion duration of acceleration (ADT), velocity (VDT) and displacement (DDT) at the alluvial sites were 1.8, 1.6 and 1.4 times respectively, longer than for rock sites. These ratios have some correlation with magnitude, distance and the subsurface structure of the Lanyang plain. While strong motion duration has a positive correlation with magnitude and distance, the correlation between duration and distance may in fact be only part of the magnitude effect. Using regression analysis on the records of 30 earthquakes recorded by the SMART1 array, we determined the relationship between strong motion duration of acceleration, velocity and displacement and local magnitude. According to Trifunac and Brady (1975), the displacement and local magnitude. According to Trifunac and Brady (1975), the variability of the duration is caused by inhomogeneous media through which seismic waves propagate, but the results of our study show that duration variability is primarily caused by the complicated rupture process of the earthquake source.