QoS-aware service selection is an active area of research on Web services composition. It is a complex combinatorial optimization problem, which solves how to find a best composition plan that maximizes user QoS requirement. This paper presents a QoS-aware Web service selection algorithm based on Ant Colony System. Firstly, a proposed Web Services Composition graph (WSC graph) is applied to model the composition problem. Then an extended Ant Colony System using a novel Ant clone rule is applied to solve the selection problem. In order to quicken the speed of its convergence, the utility function is considered as the heuristic information. Finally, the algorithm is tested for the performance.