In order to understand the formation of different crystal structures and improve the mechanical properties of isotactic polypropylene (iPP), melt vibration technology, which generally includes shear vibration and hydrostatic pressure vibration, was used to induce the change of crystal structure of iPP. iPP forms a crystal structure in traditional injection molding. Through melt vibration, crystal orientated and its size became smaller, and a change of crystal structure of iPP from a form to beta form and gamma form was achieved. Therefore, the mechanical properties of iPP were improved. At high melting temperature (230 degreesC), only beta form can be induced. At low melting temperature (190 degreesC), either beta form or gamma form can be induced, depending on the combination of frequency and vibration pressure. (C) 2004 Wiley Periodicals, Inc.