Recently interest in the fabrication of LD operated by low current is gradually increasing as fabrication techniques of MQW-LD are progressed. In this viewpoint, theoretical estimation for decreasing the amount of threshold current will be helpful to design and make LD in case that active layer of conventional bulk type RWG-LD structure is replaced with MQW structure. Therefore, the optimum design condition of RWG MQW-LD was obtained from theoretical analysis in order to operate in the weakly index-guided LD and low threshold current. The lateral effective index step has been obtained in RWG MQW-LD structure. Waveguide mechanism including this index step has been investigated by solving the carrier diffusion equation and lateral wave equation. From these theoretical results, the optimum design condition of RWG MQW-LD have been suggested.