Regard to importance of energy in the world and organizations involved in this area and the effective role of IT (information technology) for these companies to achieve their strategic goals, it is necessary to evaluate IT function in these organizations. Especially holding companies such as TAVANIR in IRAN’s power industry need to control and supervise on IT function of their subsidiary companies.IT maturity assessment is one of the evaluating ways that makes executives aware about how good their IT function works and what their maturity level is in compare with other companies. So they can choose target maturity level for their company. Then the first and most important step for promoting IT maturity level is taken and improvement projects can be planed based on the IT maturity evaluations. In this study, a comprehensive literature review of academic models and customary standards in evaluating IT maturity is accomplished, and limitations and advantages of the models are compared. COBIT maturity model is selected for this study. Six TAVANIR subsidiary companies (2 Power Generation, 2 Energy Transmission and 2 Energy Distribution companies) was assessed on the basis of an algorithm taken from the COBIT maturity model which was customized for TAVANIR. After determining the companies’ current maturity levels, Strengths and weaknesses was analyzed and improvement solutions was presented on the basis of the six maturity attributes, the process controls and the control objectives which are introduced in the COBIT model. On the basis of the evaluation, minimum and maximum level of current maturity were respectively for “Monitor and evaluate internal control “process for one of the power generation companies and “Manage service desk and incidents” process for one of the energy transmission companies. Also a self assessment process was organized for periodic and repetitious assessment of IT maturity in the holding company of TAVANIR and all its subsidiary companies.