Continuous operation of rotating machinery with a rotor crack is a risk condition since the rotor crack grows rapidly and may fail causing a catastrophic accident. This paper develops the finite element model of the rotating shaft with an open crack. The analytical method for the calculation of the natural frequencies of such a rotor system with an open crack is investigated, and the modeling of the open crack element is discussed. The natural frequency of the experimental system is measured for various cases of positions and depths of the open crack. By comparing both the theoretical and experimental results of the natural frequencies, the accuracy of the developed finite element model of the rotating shaft with an open crack is clarified.