In this paper, we analysed the ablative process by which an UV exoimer laser can remove organic material. The etch depth per laser pulse as a function of incident energy density were explained by theoretical model. For polymethyl methacrylate, two photon processes play an important role at high incident energy density. The relationship between etoh depth per pulse and absorption coefficient of the organic material were also discussed. There is a maximum of etoh depth per pulse for a special absorption coefficient.