The conditions of stability of thermodynamical equilibrium are derived by the method of statistical mechanics based on the formula of fluctuations. In the case of a chemically pure substance, the formula of energy fluctuation based on canonical ensemble is used with the result This is the required condition of stability. In the case of a homogeneous system consisting of l independent components the formula of fluctuation of energy and of the number's of molecules of different kinds are used yielding the result These are the yield conditions of stability.Finally, the condition of stability is investigated from the homogeneity rela-tion of extensive quantities.