In this study, linear homopolypropylene/clay (HPPC) nanocomposite foams with a high expansion ratio of about 18 and a high cell density of about 1.7 x 10(8) cells/cm(3) were produced using an extrusion foaming method with CO2 as the physical blowing agent. The result was much better than pure HPP foams with expansion ratios of 1.7-2.2 and cell densities of 10(3)-10(5) cells/cm(3) obtained even at the same foaming conditions. The nanoclays had a half-exfoliated structure in the HPP matrix, and their presence dramatically affected the viscoelastic properties of HPP melt and foaming behaviors. It was found that the introduction of a small amount of nanoclay significantly increased the cell morphology of HPP foams at low die temperatures, where the cell wall was very thin and cell distribution was uniform. With an increase in nanoclay content of up to 5 wt %, cell morphology was improved gradually at broader die temperatures. Based on the cell morphology results, a suitable foaming window for clay content and die temperature was established. The mechanisms behind these phenomena are discussed from the perspective of cell nucleation and coalescence. Microstructures were found in the cell walls of HPP and HPPC nanocomposite foams, and they tended to evolve with cell wall thickness, depending on the die temperatures. Scanning electron microscopy (SEM) observation of foams and solvent-etched foams revealed that the microstructures in the cell walls were formed by covering large-sized crystals and that the absence of microstructures was due to the presence of small-sized crystals in the cell walls. A distribution of crystal sizes was observed across the foamed samples, which was affected by the die temperature and the introduction of nanoclay. The possible reasons were elaborated by considerations of temperature gradient. DSC tests indicated that the foaming process induced a low-temperature peak (T-ml) and its heat of fusion (Delta H-ml) tended to evolve with the die temperature and the introduction of nanoclay.