In this paper, we report the changes of electrical, structural and optical characteristics in Ge2Sb2Te5 thin films according to an increase of Si content. The Si-doped Ge2Sb2Te5 thin films were prepared by rf-magnetron co-sputtering method. Isothermal annealing was carried out at N2 atmosphere. The crystallization speed (v) of amorphous thin films was evaluated by detecting the reflection response signals using a nano-pulse scanner (wavelength = 658 nm) with illumination power of 1∼17 mW and pulse duration of 10∼460 ns. Structural phase changes were evaluated by XRD, and the optical transmittance was measured in the wavelength range of 300∼3000 nm using UV-vis-NIR spectrophotometer. The sheet resistance (RS) of the thin films was measured using 4 point probe. Conclusivlely, the v-value decreased with an increase of Si content, while the RS-values of both crystalline and amorphous phases were increased. In particular, fcc-to-hexagonal transition was suppressed by the added Si atoms.