Cloud computing is gradually evolving as a widely used computing platform where many different web services are published and available in cloud data centers.In recent years,there has been an increasing interest in web service composition due to its importance in practical applications.For cloud computation,the key issue is that traditional service composition methods mainly focus on how to find service composition sequence in a single cloud,but not from a multi-cloud service base.It is challenging to efficiently find a composition solution in a multiple cloud base because it involves not only service composition but also combinatorial optimization.In this paper,we present a greedy algorithm Greedy-WSC and an ant colony optimization (ACO) based algorithm ACO-WSC to select a cloud combination subject to not only finding feasible composition sequence,but also containing minimum clouds.Experimental results show that a proposed method based on ACO can more effectively and efficiently find optimal cloud combinations.