The in-situ TiC particles reinforced Ti-matrix composites were fabricated by melting-casting. The mechanisms of crack forming and propagation were investigated during dynamic tension under static load by SEM. The results showed that microcracks easily appeared on the surface of TiC particles with stress concentration. The microcracks in advantageous orientation propagated continuously mainly by self-propagation and combination with ambient cracks among the micocracks in different position, forming the dominant crack by connection with surrounding cracks. When the crack propagation was baffled, new cracks formed on the front TiC particle in front of cracks or plastic pits formed in the matrix, and the cracks were connected with each other by propagating, so that the samples losed the stability and fractured rapidly after the cracks propagated to some extent. A model of crack forming and propagation was established according to the results.