Abstract: FT-IR and Laser Raman spectra of (CH_3Cp)_2Fe have been measured. On the basis of qualitative assignment the force constants of Cp_2M(M=Mn,Fe,Ru) obtained by G-F matrix have been translated to the molecule of (CH_3Cp)_2Fe, only the metal coordinate force constants have been adjusted. The same method of G-F matrix has been used for calculating the vibrational spectra frequencies of (CH_3Cp)_2Fe, the result is consistent with the observal one.