We propose a numerical model for computer simulation of macroscopic processes of diffusive phase transformations of alloys. It is ail extension of a phase field model and can treat multi-phase and multi-component alloys. This gives a general framework for Computer aided studies of many phenomena-complicated microstructure formations in alloys and intermetallic compound generation, for example. We also present some numerical results which reproduce qualitative aspects of typical phase transformations observed in real alloys. (C) 2002 Elsevier Science B.V. All rights reserved.