Usability is a critical measure of a system?s success. This study focuses on evaluating the usability of Agent99 Trainer, a Web-based multimedia training system for deception detection training. Common measures and evaluation methods for system usability are reviewed. After briefly introducing the design of Agent99 Trainer, this paper focuses on presenting the evaluation methods used in the study and the results. Both quantitative and qualitative data were collected using questionnaires for testing the following system usability attributes: learnability, usefulness, ease of use, effectiveness, and user satisfaction. The results of this evaluation indicate that Agent99 Trainer is a well-designed system with good usability. Several features that make Agent99 Trainer a useful and valuable tool are discussed, and lessons learned from this evaluation for future system improvement are summarized.