As the use of ontologies expands, their visualization is becoming increasingly important. In this study, an ontology browser for visualizing the Trajectory Simulation ONTology (TSONT) was evaluated in terms of usability by considering its subdimensions, which are effectiveness, efficiency, and user satisfaction. The methodology employed in this study for evaluating an ontology browser is reported along with the results of the evaluation. The TSONT browser is a tree-type ontology browser created to allow developers to visualize TSONT. Six flight simulation programmers with at least one year of experience participated in the study. The participants were given usability tasks and their voices and eye movements were recorded using a sound recorder and eye-tracker, respectively. The results not only showed that guidance and terminology influence the efficiency, effectiveness, and user satisfaction of the ontology browser, but they also revealed important insights into the requisites of the general usability of ontologies, even in simple text-based interfaces.