The front and rear patterning process of metal thin film irradiated by the KrF excimer laser is analyzed in this study. In the font patterning of Cu thin film with a thickness of 0.1-0.6 micrometers on polymer substrate, high speed shadowgraphs were taken by irradiating a SHG YAG laser beam co-axially with the excimer laser beam. At the optimal fluences, the molten film separated along the outer edge of the laser-irradiated region was driven toward the center of the irradiated region by the surface tension force, and the was detached from the substrate as small droplets. No explosive removal was observed in our laser patterning experiment contrary to the case reported in the literature. Under this condition, little change in the reflectivity, approximately 25 percent, was observed during laser irradiation. At the excess fluences, the recoil force of evaporation provided the outward radial flow to extend the patterning zone into the unirradiated zone so that the edge of the unirradiated region was peeled off by the momentum of the metal driven by the recoil force. As a result, the reflectivity decreased drastically in the latter half of laser pulse. In the rear patterning, the feature of the deposited metal on opposite substrate removed from the thin film was observed by an optical microscope. Intensities of incident and transmitted beam were measured simultaneously using PIN-photodiodes. Film removal started after approximately 1/100 of that of the incident beam. In the rear patterning process it was found that the recoil force of the evaporation and plasma expansion generated between the film and the substrate pressed the film subsequently, the molten part at the edge of the unirradiated part was peeled and flied away by the momentum for the recoil force.