Raman spectra of biphenyl have been obtained under pressures up to 15 GPa. The results indicated that with the increase of pressure, the effect of inter- and intra-molecular π-πconjugation and delocation increases, accompanied by the intensity enhancement of the Raman bands, and blue shift of the frequency. The intensity ratio (Rf/a) of two Fermi resonance bands υ'6 + υ' 1 and υ' 8 decreases, and the frequency difference Δ increases with the pressure. The Fermi resonance phenomenon disappears when the pressure goos up to 8 GPa. Using J.F. Bertran's theory, we obtained the relationship of the inherent frequency difference Δ0 and coupling coefficient ω with pressure. This phenomenon was explained by high-pressure phase transition. The mechanism of high pressure induced Fermi resonance weakening was also discussed.