In the trend towards developing the grid-connected distributed generation（DG） system， a study on generation expansion planning and its operation performance is of great importance to achieve optimality and reliability of power system development and operation. Aimed to minimize the total investment cost of DG and the cost of purchasing power from the main grid， this paper proposes a heuristic approach to determine optimal distributed generation capacity investment for a rural distribution system under typical transmission system scenarios， including normal operation with fixed electricity price， consideration of the outage state circumstance as well as real-time price under the competitive electric power market condition. Furthermore， through operation performance analyses under various scenarios， the influences of transmission system outage indexes， real-time price and randomness of renewable generation resource on the system design are discussed. The superiority of the grid-connected DG system is shown by the results of a numerical example.