Coalition formation is a key topic in multi agent systems. People try to search for coalition structure that maximizes the sum of the values of the coalitions, but in most cases the number of coalition structures is too large to search for the optimal one exhaustively. In this paper, an algorithm is presented that within the minimal amount of search can guarantee to find a coalition structure which is within a bound from optimum. Then, the anytime algorithm searches further, and establishes a progressively lower bound, and lowers the bound rapidly.In this stage, it evidently outperforms the algorithm presented by Sandholm etc, which is the new-made better result in this area.