A new production method for polypropylene-clay nanocomposites has been successfully developed. This method did not require the pretreatment of the clay mineral with an organo-cation. In this method we focused our attention on the nature of the clay mineral, which was exfoliated in water. Water was injected into a complex of melted polypropylene and a clay mineral in a twin-screw extruder. By controlling the pressure of the water vapor. the exfoliation of the clay mineral was achieved in the twin-screw extruder. Compatibilizers were added to the mixture of the clay mineral and the polypropylene to prevent aggregation, and we successfully prepared a polypropylene nanocomposite. The silicate layers of the clay mineral in this polypropylene nanocomposite were exfoliated and dispersed uniformly in the polypropylene matrix. This new type of polypropylene nanocomposite had almost the same properties as a conventionally prepared polypropylene-clay nanocomposite. (C) 2004 Society of Plastics Engineers.