Variation of pore structures of soils due to seismic activities affects density and fluid distribution in the pores, which in turn could affect the strength and liquefaction potential of the porous media. This paper, based on experimental investigation, studied the effect of vibrations on pore sizes of porous media. A monolayer of glass beads of different sizes under full saturation condition was used to simulate porous media, and the glass beads were subjected to the vibrations provided by a small-scale shaking table. A microscopic camera, which was positioned above the glass beads and connected to a computer, captured the pore variations during the vibration at 1sec interval. Then, graphical software was employed to analyze the changes of pore size distributions before, during, and after the vibrations. The experimental study revealed that the pore size distributions of saturated and densely packed glass beads changed little before and after the vibration. During the vibration, however, the number of bigger pores decreased and the number of smaller pores increased. It may be concluded from this preliminary experimental study that although the pore structure of dense subsoil may remain relatively unchanged before and after seismic activity, the soil could experience significant change of pore structure during the vibration, which could affect the soil's density, strength, and liquefaction potential.