Statistical mechanics is one of the fundamental papers for M.Sc. Physics and M.Sc. Chemistry courses. This book can be used as a textbook for these courses. The topics. Microcanonical ensemble. Canonical ensemble, Grand canonical ensemble, Maxwell-Boltzmann distribution, Fermi-Dirac distribution, Bose-Einstein distribution for an ideal gas. are discussed in a systematic manner by using simple language. The discussion is extended to account for interaction among the constituent particles of a real gas and the phase transitions for a matter. Finally, the kinetic and dynamical theories of gases are discussed. Each of the topics is developed in a systematic manner and is supported by exercises 1 fundamental. 2 microcanonical ensemble. 3 canonical ensemble. 4 grand canonical ensemble. 5 three distributions. 6 maxwell-boltzmann system. 7 fermi-dirac gas. 8 bose-einstein gas. 9 interacting systems. 10 kinetic theory of gases. 11 dynamical theory of gases. 12 numerical simulation.