Recently, the penetration level of different Distribution Generation (DG) units is increasing each year as due to their obvious benefits. As a result, many researchers are trying to find efficient methodologies to optimally allocate the DGs. This paper proposed a fast and accurate method for determining the optimal sizes and locations of multiple DG units to reduce losses with considering distribution systems constraints. The proposed method is applicable to solve the optimization problem of allocating different DG types. The efficiency of the proposed method is evaluated by comparing with an existing method and validated by comparing with the exact solution.