Weak interphases play a pivotal role by acting as mechanical fuse to deflect matrix cracks in future ultrahigh‐temperature ceramic fiber reinforced ceramic matrix ( UHTC f / UHTC ) composites. However, the interphase materials are not available yet. In this work, the electronic structure, chemical bonding, and mechanical properties of NiB, which is a promising interphase material for UHTC f / UHTC composites, were investigated. NiB has relatively low shear modulus of 116 GP a, moderate Young's modulus of 307 GP a, but high bulk modulus of 287 GP a. The Pugh's ratio G/B is only 0.404 and the micro hardness is predicted to be 8.2 GP a, indicating that NiB belongs to “soft” and “ductile” UHTC s. The possible slip systems are [100](001), [010](001), and [001](010) due to the presence of metallic Ni–Ni and weak Ni–B bonding. Details on the electronic structure and directional dependence of shear and Young's moduli are disclosed to highlight the mechanisms that underpin the properties.