A commercial clay, Cloisite (R) 20A (C20A) was modified with three different silane compounds, propyltrimethoxy silane, octyltrimethoxy silane, and octade-cyltrimethoxy silane, to control the hydrophobicity of C20A. The amount of the silane compounds grafted to C20A was quantified from the weight loss by using,TGA, and the hydrophobicity was evaluated by the amount of water absorption. The variation of the interlayer distance was traced by XRD and TEM observation. SEM/EDS was employed to visualize the morphology of the fractured surface and the degree of dispersion of the clay layers in the PP/clay composites. Measurements of the tensile properties of the PP/clay composites indicated that the composite made from PP and the C20A modified with octade-cyltrimethoxy silane exhibited the most excellent tensile properties. This was ascribed to the fact that the C20A modified with octadecyltrimethoxy silane possessed the most favorable interfacial interaction with PP and thus the degree of dispersion of the clay layers was the best in the PP matrix. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109:3645-3650,2008.