A newly developed cost-effective processing route, in situ reactive infiltration technique, was utilized to fabricate 47.5vol.%TiC/AZ91D magnesium matrix composites. A comparative study was made on the tensile deformation behavior at room and elevated temperatures for the as synthesized composites and the matrix alloy magnesium AZ91D and their fracture characteristics were also analyzed as well. The results show that the TiC/Mg composites fabricated by in situ reactive infiltration process possess excellent high-temperature mechanical properties. The ultimate tensile strength of the matrix magnesium alloy AZ91D at 723 K and a strain rate of 0.001 s^-1 is 41.1 MPa, while that of TiC/Mg composites at the same deformation conditions increases by 120% and reaches 91.1 MPa. At room temperature, the ultimate tensile strength of the composites increases only by 23.4% as compared with the AZ91D alloy. The SEM observation of the morphologies for the fractured surfaces reveals that the fracture mode differs from one another in that the composites are of brittle characteristic at room and elevated temperatures due to its limited failure strain, whereas the fracture mode of the magnesium alloy has a transition from brittleness at room temperature towards ductileness at elevated temperature.