In the strong electromagnetic field of a laser beam, the third-order process of radiation with two photons absorbed and a third one emitted during a transition between energy levels of a material system may become abservable. This process has analogy with the ordinary Raman effect, in which only two photons are involved. In this paper a theoretical analysis has been given with the conclusion reached, that with a beam intensity of 0.1 MW/cm2 the vibrational Raman spectrum of molecules or crystals may be obtained by the three-photon process. It is shown that this effect actually offers a new spectroscopical method. Its importance lies in the difference of the selection rule of this effect and those of the infra-red absorption and the ordinary Raman effect. It is also pointed out that the stimulated radiation of a three-photon Raman effect may be realized when pumped with a powerful laser beam.