Based on the CaCO3-B4C-C system to prepare calcium hexaboride (CaB6) powder, the influence of B4C size on the CaB6 powder was investigated in this paper, in which micro-sized B4C in various size and nano-sized CaCO3 were as main raw materials. XRD and SEM were used to characterize the phase pattern, size and morphology of CaB6 powder particles respectively. Laser particle size analyzer was employed to determine the size distribution of CaB6 particles. It was found that the size of B4C had a dominant effect on the size and distribution of CaB6 powder particles. When B4C particles were much coarser than CaCO3, the CaB6 synthesized appeared as aggregates which size relied on B4C, however, the size of every CaB6 particle was determined by CaCO3. When B4C particles size was fine to several microns, the CaB6 particles synthesized were dispersive and the size of them has great relation to B4C. Synthesis models were also established to describe various reaction processes.