The development of high strength self-healing materials with heat and radiation resistance is crucial for improving the safety and reliability of advanced nuclear energy systems. However, conventional self-healing materials—including polymers and their composites—suffer from low strength, poor thermal stability, and insufficient irradiation resistance, severely limiting their practical applications in harsh nuclear environments. Self-healing of metallic or ceramic materials at high temperatures can be achieved through oxidation, atomic diffusion, or phase changes; however, this is impractical for in-service components. MAX phases are a family of nanolayered ternary carbides and nitrides with the general formula M_n+1AX_n. Their unique combination of metallic and ceramic properties—including high-temperature stability and radiation and damage tolerance—makes them promising candidates for nuclear applications. Nevertheless, the nanoscale self-healing behavior of MAX phases, particularly the underlying mechanisms and quantitative healing efficiency, remains largely unexplored. Moreover, the mechanical properties of single-crystalline Nb₄AlC₃ MAX phase nanowires, particularly their fracture behaviors and postfracture self-healing capability, have not yet been systematically investigated. In this work, monocrystalline [0001]-oriented Nb₄AlC₃ MAX phase nanowires were fabricated using focused ion beam–based nanofabrication. The mechanical behavior and fracture surface self-healing were quantitatively investigated by in situ tensile testing within a transmission electron microscope, elucidating the mechanical properties and fracture mechanisms under uniaxial tension along the C-axis and revealing the atomistic mechanisms governing room-temperature spontaneous self-healing of fractured surfaces. An electron beam irradiation strategy is also proposed to enhance healing efficiency and understand its underlying physical mechanisms. The results show that the nanowires exhibit brittle fracture under tensile stress (fracture strength: 6.98 GPa, super-elongation: 11.5%), attributed to the size effect and absence of defects at the nanoscale. The fractured surfaces spontaneously self-healed at room temperature, recovering 30.2% of the original fracture strength. Atomic-scale high-resolution transmission electron microscopy characterization reveals that atomic rebonding across the fracture surfaces and atomic migration dominate self-healing. Furthermore, electron beam irradiation can cause lattice expansion along the C-axis and promote atomic migration, enabling more atoms to participate in rebonding and thereby further enhancing the healing efficiency (up to 56.3% recovery).