The mode-superposition method is applied to the analysis of seismic response of an inhomogeneous site which is excited by vertically propagating shear waves. The solutions for effective modal participation factor are presented for a single layer with a shear stiffness distribution exponentially increasing with depth. In an example the solutions are compared with those of a homogeneous site with an average shear wave velocity. The example also verifies that the natural frequency for the first mode can still be estimated from the formula in engineering and is an important parameter for estimating the high mode frequencies.