The explicit analytical expression for the effective correlation length ξ as a function of reduced temperature τ , external field h and system size L for the Ising-like system near the phase transition point Tc is obtained. The role of these quantities in the formation of the correlation length value is ascertained. It is shown that the irregular increase of the correlation length exists only in the case of τ → 0, h → 0 and L → ∞. With deviation from these values the essential slowing down exists for the increasing ξ. The criterium of the permissible range of temperature values (field values), where the correlation length behaviour is defined by temperature (or field) variable, is established for the fixed size of the system. Beyond this range τ < τc, h < hcr the system size becomes crucial for forming the correlation length.Key words: phase transition, correlation length, external field, finite size system PACS: 05.50.+q, 64.60.Fr, 75.10.Hk The correlation length ξ is one of the most important characteristics of the phase transition. This quantity tends to infinity with nearing to the phase transition temperature T c from exponentialat the absence of the external field for infinite systems (τ = (T − T c )/T c ). The quantity ξ 0t is called critical amplitude, and ν is (temperature) critical exponent. Now it is well established that ν depends on the system universality class (see, for example, [1,2]) and dimensionality. For Ising model, which is considered below, this value also depends on the spin component dilution (replacing magnetic atoms by nonmagnetic ones) and on the type of the impurity distribution [3,4]. In contrast to the ν the quantity ξ 0t is nonuniversal and depends on the microscopic parameters of the system. Far from the phase transition point, the quantity ξ 0t takes on the values of the order of the distance between the system particles. The value of the exponent ν as well as other critical exponents for Ising model are known with high accuracy (see, for example, [5]). These are the so-called temperature critical exponents. In addition, the field exponents that describe the dependence of the physical quantities on the field in the case of T = T c , are known as well. Since for the correlation length ξ we have the dependencewhere µ is the (filed) critical exponent of the correlation length at T = T c , ξ 0h is the corresponding critical amplitude , h = βH (H is magnetic field β −1 = kT ). Generalizing different calculation methods ( -series expansion [6], theoretical field approach for fixed dimensionality d = 3 [7], Monte-Carlo calculation data, [8], high-temperature expansions [9]), one may claim that for Ising model ν = 0.630, and µ is determined using the relations for critical exponents µ = ν β + γ .Taking into account the results of papers [6-9], we find µ = 0.402.c M.P.Kozlovskii 173