Detection and condition monitoring of a rotor system having a slant crack which occurs because of the fatigue of the shaft due to the torsional moment has been studied. Finite Element (FEM) analysis has been done for flexural vibrations by developing the stiffness matrix of a slant cracked element. Parametric study for eigen values and response has been performed with crack parameters VIZ, crack depth & location and operating and torsional frequencies. The frequency spectra of the steady state responses of the cracked rotor were found to have sub harmonic frequency components at an interval frequency corresponding to torsional frequency, which can be used for crack detection and monitoring.