Abstract: The rotor system(H858 compressor) is studied. The natural frequencies are obtained for the bending vibrations using the software "MSC.Patran & Nastran". The frequency reliability mode is defined and the equations for the frequency reliability sensitivity are mathematically expressed by using the random perturbation technique, the reliability theory and the sensitivity approach. The frequency reliability sensitivity of the rotor system is obtained by the above mentioned theory, and it is shown that the proposed method is accurate by comparing the results with the finite element results obtained by software "Nessus".