During the last decade, the phase-field method has emerged across many fields in materials science as a powerful tool to simulate and predict complex microstructure evolutions. Since phase-field methodology has an ability to model complex microstructure changes quantitatively, it will be possible to search for the most desirable microstructure by using this method as a design simulation, i.e. through computer trial-and-error testing. In order to establish this methodology, the flexible quantitative modeling for various types of complex microstructure changes using the phase-field method must first be needed. In this study, as the typical examples for the modeling of the complex microstructure changes using phase-field method, recent simulation results for the diffusion controlled phase transformations and microstructure developments in magnetic materials are demonstrated.