The thermal expansion coefficient (TEC) of nano-B4C having 50 nm mean particle size was measured as a function of applied direct current (DC) electric field strength varying from 0 to 12.7 V/mm and over a temperature range from 298 K up to 1273 K. The TEC exhibits a linear variation with temperature despite being measured over a range that is well below 50% of B4C’s normal melting temperature. The zeroth- and first-order TEC coefficients under zero-field condition are 4.8220 ± 0.009 × 10−6 K−1 and 1.462 ± 0.004 × 10−9 K−1, respectively. Both TECs exhibit applied DC electric field dependence. The higher the applied field strength, the steeper the linear thermal expansion response in nano-B4C, which suggests that the applied field affects the curvature of the interatomic potentials at the equilibrium bond length at a given temperature. No anisotropic thermal expansion with and without applied electric field was observed, although nano-B4C has a rhombohedral unit cell symmetry. The rhombohedral unit cell angle was determined as δR= 65.7046° (0.0007), and it remains unaffected by a change in temperature and applied electric field strength, which we attribute to B4C nanoparticle size and its carbon saturation.