This paper shows a new effective approach to measure crystallization temperature of soft magnetic underlayer (SUL) for next generation of heat assisted perpendicular recording media. This approach uses temperature dependent reflectivity, which shows a clear jump when samples are crystallized. To achieve this measurement, an optical system is set up using hot plate and infrared laser. Reflectivity of SUL (Co₇₀Fe₃₀)₉₂Ta₃Zr₅ shows a clear jump at its amorphous-crystalline transition temperature. Experiment results show this effect is clear in infrared region, and is weak for visible light.