Mutual injection-locking properties of a coupled pair of multiple-quantum-well distributed feedback lasers with grating output couplers were investigated experimentally and theoretically. When the mutual injection locking occurred, the output of one laser decreased while that of the other increased. The locking curve was asymmetric and a stable and an unstable locking regions existed. From the theoretical analysis, it was found that the phase delay with which the electric field emitted from each laser to the other laser significantly influences the locking characteristics. The increase and decrease of the locked output power are caused by the phase delay. It is also shown that the laser which receives the larger optical injection behaves like a slave laser and the laser which has less optical injection behaves like a master laser, and the shape of the locking curve is determined by the balance between the alpha parameter and the thermal resistance.