This dissertation proposes a method for determining load feasible region on a plane between real power axis and reactive power axis. The considered problem in this research is the transfer capability determination from a source to a sink area in a power system. The proposed method uses the boundary tracing process by determining a boundary point of load feasible region with the distance minimization from an unfeasible loading point. Then, next boundary points are traced using vector operations and the distance minimization repeatedly until obtaining a closed contour of load feasible region. The proposed method has been tested with 2-bus system, 6-bus system, IEEE 24-bus system, and Thailand power system. The results show that this method can determine load feasible regions for several source-sink pairs. The load feasible region shows limits of load changes due to several activities, e.g. load increment, load shedding, and reactive power compensation. This method can visualize feasible load conditions more obviously than the conventional load margin determination methods, e.g. repeated power flow and continuation power flow. Moreover, the proposed method can determine the outermost boundary of load feasible region affected by the considered system parameters.