In this study, the main goal is to obtain montmorillonite nanocomposites of polypropylene (PP). To achieve this goal, a two-phase study was performed. In the first part of the work, organomodified clay (OMMT) was synthesized and characterized. Octadecyltrimethylammonium bromide (ODTABr) cationic surfactant was added to the clay (Na-activated montmorillonite, MMT) dispersions in different concentrations in the range of 5 x 10(-5)-1 x 10(-2) mol/L. Rheologic, electrokinetic, and spectral analyses indicated that ODTABr has interacted with MMT at optimum conditions when the concentration was 1 x 10(-2) mol/L. In the second part, modified (OMMT) and unmodified (MMT) montomorillonite were used to obtain PP nanocomposites (OMMT/PP and MMT/PP, respectively). The nanocomposites were prepared by melt intercalation where the montmorillonite contents were 1 or 5% (w/w) for each case. The thermal analyses showed that the thermal properties of OMMT/PP nanocomposites were better than MMT/PP, and both of them were also better than pure polymer. Increase in the concentration of MMT (or OMMT) decreased the thermal resistance. Based on the IR absorption intensity changes of regularity and conformational bands, it is found that the content of the helical structure of macromolecular chains has increased with increasing concentrations of both MMT and OMMT in the nanocomposites. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 367-374, 2009