<p>Boron nitride nanotubes (BNNTs) have great potential as reinforcing agents in polymer composites due to their robust mechanical and thermal properties. BNNT-Polyacrylonitrile (PAN) electrospun composite nanofibers were fabricated with BNNT loadings ranging from 5 to 20 weight percent (wt%) using a high-shearing mixing process. The resulting fibers were then converted to hybrid BN carbon fiber via a stabilization and carbonization process without the addition of strain. The converted fibers exhibited interfacial bonding between the converted carbon matrix and BNNTs, without the need for surface activation or functionalization. At 20 wt% BNNT loading, the hybrid BNNT carbon fiber demonstrated a cohesive network of BNNTs throughout the fiber core, providing a high level of interconnectivity and an additional load-bearing structure. This was evident by the increase in tensile strength and storage modulus for the 20 wt% hybrid BNNT carbon fiber compared to the carbon fiber without BNNTs. Furthermore, we infused the electrospun carbon fiber with a cyanate ester resin to investigate the interface between the enhanced BNNT-seeded carbon fiber and the carbon fiber without BNNTs. Results show that incorporation of the cyanate resin improved the ductility of the brittle carbon electrospun fibers, reducing their tendency to fracture. The composites exhibited similar trends in mechanical strength to the fibers without the matrix, confirming a good interface between the fibers and the matrix. These findings demonstrate the potential of BNNTs as a high-performance reinforcing agent in polymer composites and provide insights into the design and fabrication of a hybrid BN carbon fiber.</p> Graphical Abstract <p></p>