Abstract: To obtain desirable beam energy repartition and enhance the quality of laser beat treatment, many optical systems have been developed in different parts of the world by means of splitting an initial beam and recombining the resulted sub-beams in the interaction plane, Because of the high coherence nature of laser,the energy repartitions of the recombined beams are made much more complicated by the interference and diffraction fringe structures. A detailed analysis of the thermal interaction between laser beam and the target materials is thus becoming more diffcult.In ref.[1],a theoretical calculation based on a simple optical system showed that the influence of the interference and diffraction fringes on the result of laser heat treatment can be neglected and provided that the intervals between these fringes are small relative to the size of the incident beam, and under such circumstances temperature field can be calculated using the energy repartition predicted by geometricaloptics, To genernalize this conclusion,the present paper gives mathematical expressions which can be conveniently used to determine whether or not the interference and diffraction fringes can be safely neglected.The results of experimental verifications are also presented.