In this paper, sensitivity analysis for the coupled axial and torsional undamped free vibration of ship propulsion shafting is proposed. The purpose of this study is to effectively and optimally design the resonance frequencies of propulsion shafting affecting barred speed range of main engine by modifying the diameters of intermediate and propeller shafts. The presented method is validated by the sensitivity analysis for the natural frequencies of propulsion shafting of two real large merchant ships. In addition, the changes of natural frequency and resonance main engine speed are discussed in case that the diameter is varied within the range regulated by the rule of shipping register.