Si-B system composites were prepared by arc melting using Si and B powders and then annealed in an argon atmosphere. Arc-melted specimens consisted of mainly SiB₁₄ and free Si. SiB₆ phase formed at the boundary between SiB₁₄ and free Si at content of 80 to 86mo1%B by the annealing at 1670K. The lattice parameter of SiB₁₄ increased with increasing annealing time due to the change of Si to B content ratio in SiB₁₄ phase. The electrical and thermal conductivity of the composites decreased with increasing annealing time, but the Seebeck coefficient (α) showed maxima at the annealing time around 5.4ks. The greatest dimensionless figure-of-merit (ZT) value obtained in this study was 0.14 at T=1100K.