According to ISO 9241-11, usability refers to the degree to which a software product can be used by certain users to achieve the specified objectives, with efficiency, efficacy and satisfaction. In a usability test, the satisfaction attribute is usually obtained from the coordinator’s perception of user behavior (gestures, postures, facial expressions, etc.). This makes satisfaction the most subjective of the attributes that define usability according to ISO 9241-11, making its estimation a challenge. In this paper we propose as a contribution a software system for the analysis of the mental stress of a user during a usability test, taking into account the variation of the heart rate (HRV). The proposed software system is intended to assist the coordinator of the test in the objective estimation of the satisfaction attribute, from the analysis of mental stress. As future work is intended to include the functionality of estimating the percentage of the user’s emotional behavior for each task. It is also intended to include to the software system other physiological variables to complement the analysis of satisfaction.