Due to the progress of the availability of thermodynamic data in the last decades, equilibrium thermodynamics provides the ability to calculate mineral stability relations in the Earth’s lithosphere (and beyond) as well as thermobarometric results. GeoPS (available at http://www.geops.org) is a windows-based interactive visualization program for thermodynamic modeling of phase equilibria. This program provides a wide range of phase equilibrium calculations and illustration facilities base on Gibbs free energy minimization method, which is done automatically with practically no user intervention. Possible applications include calculation various types of phase diagrams, isopleths, and thermodynamic modeling of phase equilibrium along specific P-T path with a progressively changing effective bulk compositions (EBC), such as fluid and melt loss, and chemical fractionation during compositionally zoned minerals growth in a metamorphic process. A novel algorithm with parallel computing makes GeoPS have high efficiency. The easy to use interface, visualization, high efficiency, reliability of GeoPS makes phase equilibrium modeling accessible to any researchers and students of earth sciences, and provides a powerful tool to understand natural systems and plan experimental work.