Phase change material (PCM) applied in the glazing structure can decrease building energy consumption and improve indoor thermal comfort by enhancing its thermal energy storage capacity. In the present work, thermal performance of a PCM-filled double glazing unit with different optical properties of phase change material was investigated numerically. The results show that optical properties of PCM play an important role in the thermal performance of double glazing unit filled with PCM, and effect of PCM phase is also strong. Effect of refractive index of PCM on the temperature of double glazing unit is weak, but the effect of extinction coefficient of PCM on the temperature and transmitted energy of double glazing unit is strong. Compared 200 m(-1) with 5 m(-1) of extinction coefficient, time to the highest temperature is 30 and 300 min earlier in liquid and solid PCM of double glazing unit, and time to the highest transmitted energy is delayed 40 min in liquid PCM double glazing unit, but is nearly same in solid PCM double glazing unit. (C) 2016 Elsevier B.V. All rights reserved.