This is a companion work to our previous study on the pressureless sintering of boron carbide (B4C). The Vickers hardness and indentation fracture toughness of B4C compacts were measured after various sintering heat treatments. Increases in hardness and decreases in indentation fracture toughness as the grain size decreased in sintered B4C were attributed to the effects of more rapid strain hardening associated with dislocation pileups at grain boundaries.