Abstract: Dynamic constitutive relationship such as Ramberg-Osgood and Hardin-Drnevich models, together with extended Masing rules, is used to account for nonlinearhysteretic behavior of soil material under earthquake-induced irregular loadings Step-by-step time integration scheme and load transfer method are combined to develop an efficientincrementally-iterative computational technique for dynamic nonlinear problems of Soils.Moreover. a truly nonlinear analysis procedure is presented for evaluating seismic responseof soil sites. According to the proposed comcepts, a computer program is worked out. Numericalcomputations are conducted to show a comparison between the solutions by trulynonlinear analysis and by conventional equivalent linearization approximation for a sampleproblem.