Six sigma provides a methodology for problem solving by using project management tools systematically. The primary aim of six sigma applications is minimizing defects. Minimizing defects will ensure the outputs and customers' satisfaction increase and the costs decrease. However, the number of the firms that state that they couldn't get the results from six sigma programs as they expected is not few. The main reason for failure of six sigma programs is not to balance the costs with gains of six sigma projects. So, evaluating the performance of six sigma projects is an important issue for the firms that apply six sigma programs.In this paper, data envelopment analysis was used to evaluate the performance of six sigma projects which is stated as critical success factor for six sigma programs. Data envelopment analysis which is based on linear programming helps to determine the efficient and inefficient decision making units by comparing the relative efficiencies of decision making units. Recently, there are some papers in the literature which apply data envelopment analysis for evaluating the performance of R&D and software projects. There are also some papers which discuss the application of data envelopment analysis in the selection of six sigma projects. However evaluating the performance of six sigma projects with data envelopment analysis was not discussed comprehensively in the literature. This paper is original by showing that data envelopment analysis which can be used in evaluating the performance of organizations can also be used for evaluating the performance of six sigma projects. The application of data envelopment analysis for evaluating the performance of six sigma projects was shown by a case study.