High density monolithic boron carbide specimens with grain size of 3–5 µm were consolidated by spark plasma sintering using identical heating conditions and Ar or N₂ atmospheres. The effect of impurities from two different raw powders on the flexural strength was revealed. The increase in flexural strength was observed up to 1600°C. Specimens consolidated in nitrogen had a higher strength than that consolidated in argon. Samples had the room temperature strength ranging from 350 to 550 MPa. The high temperature strength of our samples exceeding 400 MPa is higher than that previously reported for polycrystalline monolithic B₄C. The supporting cracking/strengthening mechanism was discussed and proposed.