The advancement on the information and communication technologies affects every field and the web sites of the public institutions as well. Public institutions make their information and services online as required by developing world. In this context e-government portal has been established to meet the demands of the citizens online. As a result the access of the citizens to the data and services more easily, quickly, and safely has been ensured thanks to the web sites of the public institutions. Therefore more emphasis on the designing of the web sites of the public institutions is necessary. A usability analysis is a necessity to evaluate and promote the usability of the web sites of the public institutions. However the notion of usability on the web sites of public institutions is ignored. This study is performed to evaluate the usability of the one of the most visited public web site; Turkish State Meteorological Service www.mgm.gov.tr. A practice designed to assign users to perform 5 tasks via web site. The users are asked to say what they are thinking loud during the practice. The number of clicks and the access time of the users are recorded. Before the practice a personal data questionnaire is applied to the users. The opinions of the users about the web site are obtained by the evaluation questionnaire performed after the practice. According to these opinions recommendations are noted. (C) 2013 The Authors. Published by Elsevier Ltd.