Microstructure and mechanical properties of epoxy resin/carbon black composites synthesized by in situ condensation polymerization of monomers in the presence of carbon black particles were investigated. SEM observation showed that carbon black particles were well dispersed in the epoxy resin matrix. The composite with 1 wt. % carbon black had the electrical resistivity of approximately 108.7 Ω•cm. The composites exhibited an a percolation threshold at carbon black content about 4 wt.%, indicating the existence of a path of percolation by connecting carbon black particles. Bending strength dramatically increased to a maximum value of 133.4MPa and then slowly decreased with content of carbon black increasing. Shore hardness increased gradually with content of carbon black and came to 23.3HD at carbon black 10 wt.%.