A lattice statistical-mechanical theory combined with a new group contribution was formulated for the unified description of thermodynamic properties of fluids. The formulation provides a simple procedure for predicting thermodynamic properties in terms of only two functional group parameters reflecting size of groups and energetic interaction between groups. The approach quantitatively describes pVT behaviors, phase transition, and phase equilibrium of various classes of single-and multi-component fluids. With a single set of temperature-dependent group parameters, predictions have been made even for the properties that are not readily accessible experimentally such as the vapor pressures of high carbon number pure alkanes and vapor-liquid equilibria of binary mixtures of any combination of such alkanes. Also, with an emphasis on engineering practices, the reliable predictability of the present model was demonstrated for vapor-liquid equilibria of hydrocarbons, supercritical fluids and polymer solutions.