Detection of delamination cracks in laminated composite structures is significant to improve reliability of these structures. For the detection of delamination cracks, real-time delamination monitoring is required. The composite laminates reinforced with graphite fiber can use the reinforcement graphite fiber itself as sensors for an electric potential method. The adoption of graphite fiber as sensors does not cause the strength reduction of smart structures induced by imbedding sensors into the structures. It was proved experimentally by the authors in their previous paper that the edge delamination crack was detected by using graphite/epoxy composites. In the present study, therefore, analytical investigations were conducted to investigate the effectiveness of the method for delamination cracks for detection of internal delamination cracks. As a result, this method was shown to be attractive for detection of internal delamination cracks, and the crack size, position and stacking sequences have a large impact on the change of electric resistance.