Cancer is one of the most dangerous diseases for people. Recently, development of array technologies makes it possible to measure thousands of genes at once, and it can be used to treat cancer. Various methods using array data to classify cancer are proposed, but there are no perfect and general methods. Ensemble method can demonstrate its ability if there are complementary set of individuals. However, it is needed methods to search the optimal ensembles, because there are so many available ensembles. We propose a GA-based method to search the optimal ensemble. In two benchmark datasets, our proposed method has shown significant result in the aspects of performance and time efficiency under the several situations.