In developing countries, firms usually have limited capital, infrastructure, and institutions for technological learning. Hence, knowledge of firms about external and internal learning mechanisms as well as using other firm’s experiences lead to reduced cost and time of technological learning. Present paper contributes to renewable energy firm’s knowledge by analyzing cause and effect relationships of internal and external technological learning mechanisms. For this purpose, internal and external technological learning mechanisms were extracted by literature review. Then, cause and effect relationships of learning mechanisms in Iranian renewable energy firms were investigated by the DEMATEL method. The results indicate that firms should use a combination of internal and external learning mechanisms to enhance their technological capability. Moreover, intramural research and development, trial and error, reverse engineering of competitor’s products, imitation of competitors, and licensing agreement have been the most frequent technological learning mechanisms used by Iranian renewable energy firms. These are the issues that need to be addressed by renewable energy firms and policymakers in developing countries.