For most of previous study on electron beam drilling (E.B) or laser beam drilling (L.B), the evaporating phenomena was assumed to be a minor parameter and could be neglected. In fact, the assumption usually leads to the overestimation of drilling efficiency and causes a significant deviation from experiment result. To improve this defect, a critical flow theory, based on the reacting pressure downward into the melting layer, is first introduced in this study; which not only successfully separates the evaporating model to convective model, but the critical transition of drilling characteristic will be also accessed. Examine the melting cavity of copper drilling conducted in E.B process, the evaporating phenomena responsible for drilled cavity had been verified to be as a prior parameter as the incident energy density is below 6×1010 w/m2. When compared with the experiment data made by Allmen , proposed theory shows an excellent agreement for copper drilling and their relative errors are no more than 30%.