Recently atomic cluster structures have been intensively studied because of their importance in physics, chemistry and material science. However, finding the lowest energy structure, which is the most stable configuration, is NP-hard. Differential Evolution (DE) algorithm is a new heuristic approach which mainly has three advantages: finding the true global minimum regardless of the initial parameter values, fast convergence, and using few control parameters. In this paper we describe a new search method that uses differential evolution (DE) algorithm to optimize the geometry of small argon atom clusters. Experimental results show that the exact global optimal configuration of argon clusters with atom number N≤ 16 can be found in a reasonable computing time, and approximate optimization can also be obtained for clusters with N=30. From their 3-D geometry structures, we can see that their optimal energy structures are highly symmetrical.