In this article, we report on an action case study on the use of mobile-assisted peer feedback to improve second language (L2) speakers' English performance. Drawing on the learning-oriented assessment (LOA) framework (Carless, 2007), the study made in-class use of a newly developed mobile app with the provision of peer feedback. The study was conducted with 25 Business School students in an English for Specific Purposes course at a Hong Kong university. A mixed-methods approach was adopted, including a questionnaire survey, a focus group discussion after the class and a teacher journal. In addition to the participants' general attitudes, two broad themes were found in relation to the participants' learning experience, namely the use of peer feedback and the affordances and constraints of technology. Results showed that students generally agreed on the positive effects of mobile-assisted peer feedback (e.g. real-time and anonymous feedback), while they recognized some limitations such as the small screen size of their phones and the limited number of given rubrics. Finally, we discuss the implications from different perspectives based on the reported themes.