For earthquake resistant design, especially against seismic motions, it is very important to estimate ground surface motions during earthquake. It is well known that equivalent linear method cannot be applied easily to seismic response analysis in cases of strong seismic motions with severe nonlinear behaviors. However, this method may be useful in practice if its applicability comes more extended with good precision to strong seismic motions of ground. In the study, shear strain levels and number of cycles of loading, which may have effects on modulus and damping factor of dynamic deformation property of soil, have been examined with the data obtained by cyclic deformation test using undisturbed soil samples. Then, the conversion ratio from maximum strain to effective strain used to decide the modulus and the damping factor in the equivalent linear method can be given simply in an expression of the strain amplitudes and the number of cycles of the seismic motions. The proposed expression in the method has shown good applicability to simulate the vertical array records of seismic observations at five sites.