Since we cannot manage what we cannot control, and we cannot control what we do not measure, the measurement of knowledge management (KM) effectiveness is a very important issue. This study aims at developing a methodology to evaluate the KM effectiveness under fuzziness in research organizations. In order to develop such a methodology and to define measures of KM effectiveness, first the relevant literature were studied thoughtfully, in the next step, the opinions of the experts, specialists, scholars and professionals in IT and KM systems and senior managers in Iran’s research organizations were gathered through in-depth interview and consultation. Linguistic data that people use in their judgments may result in ambiguities. To decrease these ambiguities, linguistic terms based on the extent fuzzy analytic hierarchy process (FAHP) - as a multi-attribute decision making approach- are used in this study. 34 measures, which are the major variables to evaluate the KM effectiveness, have been defined under six main measures: Human resources, leadership and organizational structure, knowledge creation and acquisition, knowledge storage and security, knowledge sharing, and knowledge utilization and updating. The results indicate that “human resources” is the most important measure. The other 5 criteria in order of importance are the “Knowledge Sharing”, “Leadership and Organizational Structure”, “Knowledge Utilization and Updating”, “Knowledge Creation and Acquisition” and “Knowledge Storage and Security”. Finally, to test validity and reliability of the proposed framework, we have used this framework to evaluate the KM effectiveness in 9 of Iran’s research organizations.