M-learning as an emerging and immature field of study has challenges that it still has to overcome. Some are pedagogical, technical, ethical, and financial; others concern its sustainability and security among others. Usability is a quality requirement of an M-learning tool. Yet very little has been reported about this aspect. This paper focuses on the evaluation of the usability of an M-learning tool. The aim is to come up with recommendations on how to evaluate such a tool.  Using the Goal Question Metric (GQM) approach, a proposition of M-learning User Interface evaluation metrics and measures is made. These metrics can be used to collect qualitative and quantitative data.