The use of geographical data is increasing in real world applications and due to this, Geographic information systems (GIS) are a growing interest area for research nowadays. Today, GIS is combined with the World Wide Web (WWW) to provide information to a lot of people. Web-based GIS applications have gained popularity because of their easy use and simplicity. Through browsers, web-based GIS applications can display maps with features like buildings, forests, roads and even traffic. Furthermore spatial data can be analyzed and large amounts of data can be viewed on small screens by zooming and panning the map. So having all the above features, effective web-based GIS applications should be designed in a way that they should be easy to use and understand for users. Usability evaluation is becoming an important part of Interaction Design to find out the user´s needs and requirements. In this study two web-based GIS applications (Google Maps and MapQuest) are evaluated to find out usability problems. To accomplish the main goal of this study, different techniques are used. Special criteria are adopted for usability evaluation of these two web-based GIS applications. Usability tests have been performed with six graduate users and questionnaires were filled out by these users. Interviews were conducted with six users to validate the results. The authors have provided a number of suggestions based on the problems that were identified in this study for Google Maps and MapQuest to improve the usability of these applications.