BACKGROUND Poor ergonomic design of ventilators can result in human errors. In this study, we evaluated the ergonomics of ventilators through respiratory therapists' performance, workload, and user experience. MATERIAL AND METHODS Sixteen respiratory therapists were recruited to this usability study of 3 ventilators. Participants had to perform 7 tasks on each ventilator. Respiratory therapists' performance was measured by task errors of all tasks for each participant. Workload was measured by objective measurement (blink rate and duration) and by subjective measurement (NASA-TLX). User experience was assessed by the USE Questionnaire. RESULTS For task errors, significant differences were found among ventilators (p<0.05) and the Evital 4 received higher task errors when compared to the Servo I (p<0.05). For blink rate, significant differences were found in tasks of starting the ventilator, ventilator monitoring values recognition, ventilator setting parameters modification, alarm parameter recognition, and resetting among ventilators (p<0.05). Furthermore, blink duration was also found to be significant differently in tasks of starting the ventilator, mode and setting parameters recognition, ventilator monitoring values recognition, ventilator mode modification, and alarm parameter recognition and resetting, as well as in the average of all tasks (p<0.05). For perceived workload, the Evital 4 received higher NASA-TLX scores among ventilators. For user experience, the Servo I received the highest scores on the USE Questionnaire among the ventilators. CONCLUSIONS The study provides a comprehensive evaluation method of user interface based on respiratory therapists' performance, workload, and user experience. In addition, this study suggests that the ergonomic design of the Evital 4 is poor. Finally, we found that eye motion (blink rate and duration) may be useful to assess the ergonomics of a user interface.