Display omitted •Single PI/rGO composite nanofiber by in-situ polymerization and in-situ thermal conversion.•Tensile strength of single nanofiber up to 4.2 GPa (PI/rGO-1.0% single nanofiber).•Modulus of single nanofiber up to 121 GPa (PI/rGO-1.2% single nanofiber).•Homogeneous dispersion of rGO by in-situ strategies.•In-situ strategies enhance the interfacial interaction between rGO and PI fibrous matrix. High mechanical performance electrospun polymeric nanofibers are highly desired for practical applications, especially as reinforcements for composites. However, most of the electrospun polymeric nanofibers present tensile strength < 3 GPa. To overcome such limitation, this work successfully prepared single rGO reinforced polyimide composite nanofiber with tensile strength up to 4.2 GPa (PI/rGO-1.0%) and modulus up to 121 GPa (PI/rGO-1.2%) by applying in-situ polymerization, electrospinning, and in-situ thermal conversion. These mechanical properties are higher than other polymer-based electrospun nanofibers, and 45% and 236% higher than those of neat PI single nanofiber, respectively. The in-situ strategies provide the homogeneous dispersion of rGO in single electrospun nanofibers and enhance the interfacial interaction between rGO and PI. In addition, the PI/rGO composite nanofibers also present excellent thermal stability with glass transition temperature (Tg) > 295 °C, and the 5% thermal decomposition temperature (T5%) > 539 °C. This work would open a new route for the preparation of high performance electrospun nanofibers for composites.