A reversible insulator-to-metal transition in phase change materials is expected to be usuful for a variety of optoelectronic devices including terahertz (THz) applications. Here, we report on the phase-dependent changes in the optical properties of the thin films of Ge2Sb2Te5 (GST) phase change material. THz time domain spectroscopy measurements were carried out for amorphous and crystalline samples obtained by annealing at different temperatures and the optical properties of the GST films in the THz frequency range were characterized. It was found that the amorphous GST film was almost transparent to THz radiation due to the of absence of free carriers. On the other hand, the crystalline GST films were found to absorb THz radiation depending on the annealing temperature. The index of refraction was found to enhance with increasing annealing temperature. These properties are promising for THz wave devices such as programable amplitude modulator or plasmonic devices.