For simultaneous prediction of solid-liquid and liquid-vapor phase transitions it has been customary to apply two different theories for solid and fluid phases. A single-theory approach will be desirable to answer many of the fundamental problems of molecular theory and their relationship with macroscopic behavior of the matter. Based on a modified version of the cell model of statistical mechanics, a single-theory approach for simultaneous prediction of solid-liquid and liquid-vapor phase transitions is presented here. In developing this theory the order-disorder transition is considered as the essential feature of the fusion and a new function for the potential energy field inside a single-occupancy cell is derived, By reporting the variations of total pressure of the macroscopic system with respect to temperature and volume the nature of the various phase transitions in the system are evaluated and discussed. Variations of the radial distribution function of the molecules in the system with intermolecular distance, temperature, and volume are reported for various phases of matter.