Abstract: The conformations of title compound in the crystal and in the isolated state have been calculated by means of QCFF/PI+MCA (Quantum Mechanical Extension of the Consistent Force Field to PI Electron System+Molecular Crystal Analysis) computer program. The results of investigation indicated that the twist angle between the two phenyl rings of biphenyl in the molecule is about 18.9° in the gas phase and 1.3° in the crystalline phase. The conformation of the molecule in the crystal is in agreement with that obtained by X-ray crystal structure determination. We have calculated equilibrium geometries and vibrational frequencies of 4,4'-diben-zoylbiphenyl. The observed and some calculated vibrational frequencies are listed in Table 1.