This paper presents a new methodology that can evaluate transfer capability of composite power systems from the adequacy point of view in power system planning stages. First of all, to evaluate practical load supplying capability, nonlinear optimization problems of maximum load supplying capability(MLSC) and economic load supplying capability(ELSC) are formulated and solved by nonlinear primal-dual interior point method. Here, physical constraints considered in the optimization problems are the limits of bus voltage, line overloading, and real & reactive power generation. Also, an evaluation method of transfer capability is presented based on margins calculated by the MLSC and ELSC. Especially, to evaluate transfer capability flexibly, simple indices such as expected MLSC, transfer capability margin, and power not supplied are respectively proposed by considering (N-1) line outage probability. Numerical results on IEEE RTS 24, IEEE 118, and IEEE 300 bus system show that the proposed algorithm is effective and useful for power system planning stages.