Four different kinds of TC18 Ti matrix composites containing trace TiB and TiC are fabricated by in-situ synthesis method in this work. The experiments show that the merging of β grains is accelerated with increasing the reinforcements during heat treatment, and the standard deviations of β grain size distributions tend to decrease. The microstructure phenomenon is associated with the influence of the reinforcements on the boundary migration of β grains. It is revealed that the decrease of the pinning capability of the reinforcements on the grain boundary migration plays the most important role in the microstructure refinement law. The plasticities of the composites are excellent during tensile test. However, when the molar ratio of TiB to TiC in the composites is 4:1, the yield ratios of the composites are all higher than 0.96, and gradually increase with the increase of the reinforcements. The grain boundaries absorb the dislocations, which slows down the increase of dislocation strengthening during the strengthening process. It is suggested that the reinforcements accelerate the absorption of dislocations, and thus lead to this result. Just because of this, the contribution of the microstructure refinement to the toughness increases with the increase of reinforcements.