Policy evaluation is essential for governments to improving effectiveness and accountability. In science, Technology, and Innovation (STI) domain in line with the growth of national and regional policymaking, the literature and experiences of policy evaluation have been developed. In this paper, after reviewing concepts and various components of policy evaluation including criteria, timing, methodologies, and indicators a complete evaluation process with its all modules is presented. The main criteria including appropriateness, efficiency, and effectiveness with their indicators, quadruple timing including ex-ante, real-time, intermediate, and ex-post evaluation, triple levels of aggregation including policy, program, and activity as well as quantitative, qualitative, and semiquantitative methodologies are mentioned as evaluation components that prepare necessary information for evaluation expert groups in a systematic process. The results of policy evaluation are always published and provide feedback to policymakers for improving the goals and programs. The paper also discusses the evaluability of policies and at the end presents an evaluation experience for nanoTechnology policies in Iran.