Recent analysis of array observations indicated that pore-water saturation of soils may strongly affect vertical site amplification. A study is therefore motivated to investigate the saturation effect on seismic reflection from a boundary of porous soils. The problem described herein corresponds to a P- or SV-wave incident at the interface between rock formation and overlying sand. The sand is modeled as a partially water-saturated porous material while the rock is approximately regarded as ordinary one-phase solids. Preliminary results show that in either P- or SV-wave incidence, even a slight decrease of the complete saturation may lead to a substantial influence on both reflected and transmitted waves as well as the amplitude ratios between horizontal and vertical motion at the interface, and this influence is dependent on the angle of incidence. The underlying suggestion is that we may need to carefully consider the saturation effect in the interpretation of field observations, especially in such situations that partial saturation may very probably take place.