Polypropylene (PP) foam has been considered as a potential substitute for other thermoplastics foams in industrial applications. However, the key concern is the weak melt strength of PP, which leads to a high content of open-cell structure in PP foams; and, thus, unsatisfactory for a number of applications. In this work, PP was modified by grafting with unsaturated linear polyester (ULP) in a twin-screw extruder in attempt to improve the melt strength of PP. The grafting reaction on PP and the modified PP were characterized using FT-IR, DSC, and TGA. The improved foamability was verified by SEM observation. In addition, the rheological behavior of modified PP was investigated using a Hakke rheometer. The results indicated that the melt strength of grafted PP was significantly enhanced, facilitating the foam formation. (c) 2006 Wiley Periodicals, Inc.