TiB2-TiC ceramic matrix composites were fabricated by in situ synthesis under hot isostatic pressing (HIP) with 3TiH2-B4C and 11TiH2-3B4C being the raw materials, respectively. The XRD analysis indicates that the samples synthesized from 3TiH2-B4C have TiB2 and TiC phases, while those from 11TiH2-3B4C have not only TiB2 and TiCx phases, but also Ti3B4 phase. TiB2-TiCx ceramic matrix composites obtained from 11TiH2-3B4C have better mechanical properties than those from 3TiH2-B4C. The microstructures of the composites are investigated with SEM and TEM. TiB2 grains are platelet grains; Ti3B4 grains have two different morphologies. Ti3B4 phases are formed through two different paths: fine grain Ti3B4 is formed through TiB+B=Ti3B4 at low temperature; large plate-like grain Ti3B4 is formed through TiB2+TiB=Ti3B4 at high temperature.