To provide a carbon fiber reinforcement having excellent thermal conductivity and mechanical properties which is manufactured by mixing together two different types of pitch-based carbon short fibers having a ratio of the degree of filament diameter distribution to average fiber diameter of 0.05 to 0.2 and a fiber length of 20 to 6,000 .mu.m which differ from each other in average fiber diameter or by mixing one of them with a pitch-based carbon fiber web to improve dispersibility into a matrix resin or increase the dispersion ratio of the pitch-based carbon short fibers.