Fast and low power consumption optical switches are required for photonic networks. To this end, we proposed the optical gate switch using phase change material (PCM) and silicon waveguides. This switch had low power consumption because it consumed power only when the state was changed. Furthermore, the chip size is very small due to the large refractive index change of PCM. In this paper, we studied the phase-change characteristics of various kinds of thin GST film on SOI (silicon-on-insulator). A laser diode (LD) with a wavelength of 660 nm was used to irradiate the material. We compared the optical responses by laser pulse irradiation on GST films, and concluded GST-147 was the most suitable material for the optical switch because it had the lowest phase change threshold. The phase-change characteristics of GST-225 films with thickness of 25 nm, 50 nm and 75 nm were also examined. Thicker GST films had lower phase change thresholds. However, thermal simulations showed that the phase of the bottom part of thicker films may not be changed. Therefore, we concluded that GST films with thicknesses between 25 nm to 50 nm are the most suitable for optical switches.