Bending experiments were carried out to investigate mechanical properties and failure processes of composite T-joints with two defects. The defects of interfacial debonding and fillet-filling ratio were introduced to induce different failure modes of the T-joints. Results suggest that a great diversity exists among the T-joints under various failure modes. The maximum bending load of the intact T-joint is 288.5N, while interfacial debonding will reduce its carrying capacity by 30%. The reduction of the fillet-filling ratio leads to the initiation and propagation of the cracks in the fillet, which greatly decreases bending mechanical properties of the composite T-joints.