Mechanically strong electrospun nanofibers at high temperature are highly desired in aerospace industry, high temperature filtration and fire protection clothing. In the present work, highly tough poly(imide-co-benzoxazole) (PI-co-PBO) nanofiber belts with excellent thermal stability, mechanical properties and thermal mechanical properties were produced from the high viscous methoxy-containing polyamic acid (MeO-PAA) solution by electrospinning followed with thermal rearrangement. The chemical structures of the polymer nanofibers and the corresponding thermal rearrangement were confirmed by 1H-NMR, FT-IR and TGA. The mechanical properties and thermal mechanical properties were characterized by tensile test and thermal mechanical analysis (TMA). The aligned nanofiber belt heat-treated at 450 °C (ANF-450) had the highest tensile strength of 559 MPa, modulus of 11.2 GPa and toughness of 12.0 J g-1, respectively, which were much higher than those of the pure PBO films made by thermal rearrangement. The TMA test showed that the ANF-450 exhibited high tensile strength retention of 90% and modulus retention of 94% at a high temperature of 200 °C. These excellent thermal mechanical properties make electrospun PI-co-PBO nanofibers promising candidates in high temperature areas.