Diagnostic assessment (DA) has its objective in generating fine-grained diagnostic profiles for effective treatment or intervention. Most existing DA studies on English academic writing focus on developing diagnostic tools to identify problems or difficulties within written texts. Rarely are there studies that design and implement a treatment or intervention phrase following the diagnosis. Moreover, despite of its importance, few studies have examined the construct of discourse competence. This thesis project, therefore, firstly diagnosed EFL learners’ development of discourse competence (DC) as indicated by their academic writing in university, and then based on the diagnostic results, designed and implemented an intervention study.The project involved six Chinese EFL learners studying in a university in Hong Kong, nine individuals studying in mainland, and their 277 peers pursuing their first degree in an international degree program based on a comprehensive university in mainland China. In both contexts, the participants need to write academic assignments in English. Methodologically, the project adopted a mixed-method approach and was conducted via four inter-linked studies, namely, a text analysis study, a comparative multiple-case studies, a survey, and an intervention study. The diagnosis phrase, consisting of text analysis, multiple case study and a survey, examined student writers’ textual features, composing strategies, and academic writing knowledge status. The intervention phrase was designed to resolve students’ problems and difficulties identified in the first phase.The results showed that the cyclical diagnostic assessment procedure was effective in identifying specific lacks, problems and difficulties in learners’ written products, composing process and knowledge status, and the diagnosis could inform the design of intervention measures in teaching and learning of EFL academic writing. Specifically, contextuality was found to be a critical element for