The theory of static critical phenomena in systems at equlibrium is discussed. The nature of the classical phenomenological theory of Landau is explained. Critical point exponents β, γ, γ', δ, . . . obtained from the Landau theory differ from the results of the theory for Ising model and experimental results. Statistical evaluation of the order parameter is obtained by taking into account the fluctuation. Then the correlation effect is considered. These approaches make implicit use of the assumption that the free energy if expandable in power series in the order parameter. From the calculation of spatial fluctuation and long range correlation effect the validity of the Landau theory is discussed.