Despite extensive efforts devoted to developing self-healing materials in the past half-century, very limited successes are reported for ceramics or metals. Reported self-healing materials usually have low healing strength (megapascal) and long healing time (hours), and the healing of ceramics or metals normally requires external stimuli. Here, we report on intrinsic, highly efficient self-healing phenomena in Ti3AlC2 MAX phase nanowires at room temperature, which exhibit both ceramic and metallic properties. In situ transmission electron microscopy tensile testing reveals that the fracture strength of 2.1 GPa is achieved on the fractured Ti3AlC2 nanowire after self-healing for 5 min, corresponding to the self-healing efficiency of 36.2%, and the smaller the diameter, the higher the self-healing efficiency. The underlying mechanisms are uncovered by atomic-resolution characterizations combined with atomic simulations. The highly efficient self-healing of Ti3AlC2 is attributed to the cleavage behavior, atomic migrations, and rebonding on fracture surfaces. Al atoms trapped between partially filled Al layers on both fracture surfaces act as obstacles for the TiAl rebonding and are responsible for the size effect. These findings provide new insights into developing high-performance micro- or nano-devices, especially those that require high security and long service lifetime.