This study examined the usefulness of feedback provided by a diagnostic reading test for English learners at the tertiary level in China. A three-phase study was designed to uncover (1) key components of diagnostic reports, (2) teachers’ and students’ interpretation of diagnostic feedback, and (3) their use of diagnostic feedback and the impact of feedback on teaching and learning. In phase one, key features of diagnostic feedback provided by the score reports were coded based on Roberts and Gierl’s (2010) and Alderson’s (2015) frameworks of classification of feedback. Phase two involved questionnaire data collected from 487 students and interviews with 12 students and 5 teachers to gain insights into their evaluation of diagnostic feedback. In phase 3, 2 teachers and 8 students participated in a longitudinal case study in which they were interviewed and completed monthly reflective tasks throughout a semester in a way to examine the use and effect of diagnostic feedback in the classroom context. The results showed that the score reports incorporated various types of feedback suggested in the literature. Students and teachers were on the whole positive about diagnostic feedback and also showed individual preferences towards particular types and modes of feedback. The case study provided evidence of teachers’ and students’ remedial activities. In addition, it brought up the issue of how diagnostic competence influenced the use of diagnostic feedback and also highlighted the importance of aligning diagnostic feedback with the curriculum so as to maximize its beneficial effects.