Abstract: A new approach is briefly introduced for the optimum estimation of the thickness of erosion using primary porosity data．There are two keys that should be considered in this method ：(1)the porosity data must be primary；(2)the porosity on the ground surface is not a fixed constant，but a variable changing with the sedimentation condition and other factors. In this paper we deal with these two aspects separately．For the former，we revise the porosity based on study of diagenesis．For the latter，optimum statistic principle is used. The method is applied to both the mudstone and sandstone from Mingshui Formatlon of Changling Depression in southern Songliao Basin. Both two estimations yield a similar result of about 500 m of erosion by using computer program．Considerations that should be taken into account in geological application are also discussed in this paper．