The effect of kaolin content on the cell structure of PS foam and the correlation between the cell structure and the physical properties were evaluated in this work. The kaolin as a nucleating agent did facilitate the cell nucleation. The cell structures were apparently enhanced with increasing kaolin content. In addition to facilitating nucleation, the kaolin confined the motion of chain segments of the matrix polymer, which contributed to the change in cell structure by confining cell growth and cell coalescence to some extent. The foam with smaller cell size and larger cell density absorbed more energy, hence the impact strength increased remarkably with decreasing cell size and increasing cell density.