This work proposes forming-free HfO2-based resistance random access memory (RRAM) using X-ray irradiation which overcomes the bottleneck of the conventional forming process, that of the need for a larger forming voltage with scaling. In addition, a large irradiation area is beneficial to the mass production of memory devices. This forming-free device has a better memory window compared to the device without X-ray irradiation. Moreover, the forming-free device has good reliability as the device without X-ray irradiation. To analyze the physical mechanism, the conduction mechanism was investigated by the current-fitting technique, and irradiation experiments with varying X-ray energies and times were performed. Finally, a physical model is proposed to explain the mechanism of the forming-free RRAM from X-ray irradiation.