Abstract: A method is proposed to calculate vibrational frequencies and intensities of an adsorbed molecule system with P2mm or P4 symmetry. The method can be used if the system can be considered as a group of coupled oscillators with one freedom per oscillator. The stretch vibrations and lateral interactions of CO molecules adsorbed on Cu(100) face are studied by means of this method. The calculated results show that the lateral force constants have a strong dependence on interaction range assumed. The size effects on vibrational frequencies of adsorbed system are also examined. The results given here are to be expected.