Ge 2 Sb 2 Te 5 -TaO x ͑GST-TaO x ͒ composite films were used as a medium for phase change memory ͑PCM͒. As TaO x concentration was increased from 0 to 38.3 mol %, the activation energy for crystallization was increased from 2.01 to 2.40 eV. Compared to GST ͑6.8%͒, the GST-TaO x ͑32.2 mol %͒ film had a lowered density change during crystallization. Residual stress of the GST-TaO x composite films was reduced by 40%. This improvement was attributed to smaller grain size of the GST-TaO x films, which facilitated the relaxation of the stress via grain boundary diffusion or sliding. Compared to pure GST PCM cell, reset voltage of the GST-TaO x cell was obviously reduced, which was attributed to the reduced thermal conductivity by incorporating TaO x into GST. Meanwhile, PCM cells based on the GST-TaO x composite films possess promising endurance characteristics, implying that they are potential candidates for random access memory.