Boron carbide samples exhibiting different microstructures were sintered by Spark Plasma Sintering. Two commercially available powders were used the TK powder, which is composed of nanometric grains and the HD one, which is characterized by submicronic particles. Both powders have a different chemical composition (B/C and B/O ratios) and their stoichiometry are evaluated to B3.82CO0.05 for HD and B4.59CO0.18 for TK. The mechanical properties of sintered materials, having the same relative density (i.e. 96 percent), were characterized at the granular and macroscopic scales. At both scales, even if the TK material had significantly finer grains (i.e. 82 vs. 474 nm for TK and HD materials, respectively), its rigidity and hardness is reduced by a factor between 10 to 20 percent, mainly due to a higher content of oxygen. This study demonstrate the necessity to control the chemical composition of boron carbide powders in order to produce boron carbide ceramics with improved mechanical properties.