To meet the needs of instance aspect handling in practical workflow applications, a model for instance aspect handling- oriented optimal scheduling of multiple activity instances is constructed. An algorithm for such scheduling optimization is presented correspondingly. It utlizes the theory of ant colony optimization to achieving the objectives of minimum acitity instances' total dwelling time and minimum acitity instances' total cost with constraints. The conception of wasted grouping time and wasted grouping cost are introduced according to the two optimization objectives, based on which the heuristic information for the ants are designed. The result of simulation experiment shows its effectiveness.