Usability evaluation of collaborative writing system for education usage is essential to improve its functionality. In this paper, the usability of web office (ThinkFree doe) is tested using mix research approach. Usability evaluation is performed via the think aloud protocol. A questionnaire was designed for quantitative survey. The questionnaire was completed by users who have been using Thinkfree doc for educational collaborative work. Interviews were conducted with the selected participants for results validation. During usability evaluation process, positive and negative effects regarding software's usage were recorded. The results revealed that the overall systems response is very slow. The software needs to improve its processing speed in order to make it more efficient for future use. The system also needs to improve overall functionality (for example collaborative work, synchronization, uploading and track changes) to provide accurate and complete results.