Abstract: Based on modern organic metamorphism theories, maturity of organic matter (often expressed as vitrinite reflectance Ro ) can be modelled as a function of geologic time and paleogeotemperatures ( i.e., the thermal history): M=F(t, T). ( 1 ) When the paleogeothermal gradient is estimated, the thermal history varies with the erroded thickness X: T = f (X, t) ( 2 ) Thus the model predicted maturity M is 8 function of the eroded thickness.If a series of vitrinhte reflectance data below the unconformity are available in a well eg. (Roi, i = 1, 2,……, m ), an object function can be setablished as: I=Σ[Rc(Mi)-Roi]2( 3 ) where Rc(Mi ) is the calculated vitrinite reflectance from predicted, maturity corresponding to each measured ROi.Therefore I is obciously a function of the eroded thickness.The computer program is deaigned to search for an optimal X which gives the smallest value of I, ie.. the best fit of the calculated maturity to the measured. Application of the method in two actuat wells is given to show the validity of the method.