Abstract: The single back scattering model proposed by Aki is convenient due to its simplicity. This paper shows that the multiple scattering model whtch is an imporovement upon .Aki's model also gives a very simple and successful approach to estimate the heterogeneities under a certain area. Both the intensity and the decay of codas were used to find the Q-tactor related to scattering. It is noted that although both the single scattering model and the multiple scattering model give quite close results, the results from the latter are more reasonable and self-consistent. This paper also suggests that whichever simplified model is used, the logarithmic term in its equation is not negligible. As an example, seismograms of local erthoquakes recorded at the Beijing Regional Seismological Network were used and the mean free path of S-vvave under the Beijing area was obtained at frequencies around 1 Hz, shown in Table 2.