Dense [boron carbide (B4C)]/[carbon nanomaterials] composites were synthesized and sintered simultaneously using pulsed electric-current pressure sintering (PECPS) at 2173 K for 6.0×102 s (10 min) under 50 MPa in a vacuum. The starting powders were amorphous B and C nanopowders and nanocarbons. The latter were acid-treated carbon nanofiber CNF and carbon nanotube CNT. The sintered composites were evaluated from the viewpoints of mechanical properties at high temperatures up to ~ 2023 K in inert atmosphere. Thus fabricated composites with 10vol%CNF maintained high bending strength σb around 750 MPa even at 1973 K; this temperature is 100 K higher than that of conventional B4C/CNF composites, and furthermore 600 MPa at 2023 K. These high σb at elevated temperatures might be explained by both the low content of catalytic Fe particles and the rough surface of CNF after the acid-treatment. On the other hand, B4C/CNT composites displayed 770 MPa at 1723 K. The stress-strain curves demonstrate that B4C/CNF composite deformed elastically until 1273 K and plastically up to 1973 K, however, the B4C/CNT composites displayed elastic deformation up to around 1873 K.