In ZMR of SOI structured polycrystalline silicon thin films using tungsten halogen lamps, a heat equation with moving heat source was numerically solved. A new physical model which can describe the solidification behavior of silicon thin film was proposed. From the simulation results, the supercooling in liquid phase due to the difference of reflectivity and emissivity of silicon is expected and the interface is expected to be unstable. As the beam intensity and the scanning velocity increases, the interface morphology is expected to change from nonplanar to planar and the resultant defect density is expected to increase. Experimental results agreed well with those theoretical model. By introducing the interface instability analysis, we proposed the process diagram for the first time as a function of beam intensity and scanning velocity.