Abstract: A new scheme based on the fundamental postulation in statistical mechanics for perturbating molecules to create the Markov chain in Monte Carlo calculation was proposed. The new method can be used to simulate, in only a single box, phase equilibria of fluids including interfaces. Furthermore, for pure fluids, simulation can be implemented simply in the canonical ensemble. Simulation of pure square - well fluid at different temperatures by applying our new method gave the coexistence densities of bulk phases, the density profiles and chemical potentials along the z axis. In addition, our present study shows that the traditional Metropolis technique is not suitable to the simulation of such heterogeneous systems.