Abstract: We have derived the linear relations between the logarithm of duration of coda waves lg(tDu) and surface wave magnitude Ms for not very large duration, based on the theory given by Aki (1969). He suggested that the coda is a result of superposition of backscattered waves from numerous randomly distributed heterogeneities in the crust and upper mantle.From the statistical results of data of 129 earthquakes recorded at 21 seismic stations the following relations are obtained:Ms = 2.0lgtDu - 0.6 (103Du 3s,5.4 s s = 1.5lgt + 0.9 (60 3s,3.5 s= 2.0lgtDu - 0.5 (103Dul3s, 5.5 s s = 1.5lgtDu + 1.0 (3.5 s s can then be quickly determine directly from the duration according to the above relations.