The microstructure and optical properties of amorphous and crystalline Ge2Sb2Te5 thin films prepared under different sputtering conditions were investigated. The microstructure of amorphous films was modified by changing the sputtering Ar gas pressure during the deposition. The optical properties and the microstructure of the sample prepared at high Ar gas pressure were remarkably different from the samples prepared at low pressures. A strong correlation between the microstructure and optical properties of Ge2Sb2Te5 thin films was found. © 1999 American Institute of Physics.