In this study, the viscoelastic behavior of a microcellular plastic was investigated with particular emphasis on the effect of cell size. A poly(ethylene terephthalate) resin containing a polyolefin nucleating agent (CPET) was selected as the test material. In order to investigate the effect of cell size, microcellular CPET samples were produced having a nearly constant density of nucleated cells and a varying cell size. The viscoelastic behavior was investigated using a dynamic mechanical analyzer in tensile mode, and the relationship between viscoelastic behavior and cell size is discussed.