Phase transformations in a Ge1Sb2Te4 system induced by a single femtosecond laser exposure were investigated. The system has a multilayer structure of air/10 nm ZnS-SiO2/80 nm Ge1Sb2Te4/80 nm ZnS-SiO2/0.6 mm polycarbonate substrate. The morphology and contrast of marks written in both amorphous and crystalline backgrounds by single femtosecond pulses were characterized using an optical microscope. X-ray diffraction (XRD) was applied to identify the crystal structures transformed by single 108 fs shots. The characteristics and the conditions of phase transitions in the multilayer structure triggered by single shots were investigated. The pulse energy window for the crystallization in the Ge1Sb2Te4 system was established. The mechanism of phase change triggered by 108 fs laser pulses was discussed.