Blending polypropylene (PP) with elastomeric modifiers provides a simple method of improving polymer's impact strength. Such PP/elastomer blends are commonly called thermoplastic polyolefin (TPO) blends. Developing TPO materials suitable for foaming is of great interest because they can be applied in high-volume markets such as the automotive industry. For immiscible polymer blends such as TPO, it has been often noted that the dispersed particles can act as cell nucleating agents, there by enhancing heterogeneous nucleation. However, little work has been done to assess the effects of blend morphology on the nucleation behavior. Furthermore, the effects of elastomer dispersion on TPO foamability are still unknown. In this work, TPO blends with different blend morphologies were prepared by controlling the viscosity ratio between the blending components. Experimental results from both batch foaming and extrusion foaming processes with nitrogen (N-2) indicate that the foam structure is influenced by the size and the number density of the dispersed particles. (C) 2011 Published by Elsevier Ltd.