Reaction sintering of sodium carbaboride (NaB5C) was performed by heating compacts of mixtures of amorphous boron (B) and carbon black (C) powders (B/C molar ratio, 5/1) at 1173 K in Na vapor. The ceramics obtained from the compacts of B-5/C powder mixed with a ball mill (compact density, 1.67 Mg/m(3)) were the composites of NaB5C (74 mass%) and unreacted B-5/C (26 mass%). The bulk density and bending strength of the composite ceramics were 2.04 +/- 0.03 Mg/m(3) and 320.9 +/- 10.4 MPa, respectively. Transmission electron microscope observation revealed that nanometer-size amorphous grains of B or C were included in the matrix of NaB5C.