Encourage people to investment in small-scale electricity generation and expansion of dispersed generation (DG) may have several advantages such as, reducing the needs for investment in power plants and transmission network developments, improving the competitiveness of electricity market and moderating the costs of electrical energy procurement. In this paper, an encouraging market rule in a pay-as-bid system is proposed to accelerate the appropriate investments in DG units. The proposed rule is viewed and analyzed from the perspective of DG investor and Independent System Operator (ISO) using a novel DG placement and sizing algorithm based on Monte Carlo and Genetic Algorithm (GA). The proposed algorithm is implemented on IEEE 30-bus test system under the encouraging rule and the simulation results are presented and discussed.