The study utilised a fine-grained diagnostic checklist to assess first-year undergraduates in Hong Kong and evaluated its validity and usefulness for diagnosing academic writing in English. Ten English language instructors marked 472 academic essays with the checklist. They also agreed on a Q-matrix, which specified the relationships among the checklist items and five writing subskills. This conceptual Q-matrix was refined iteratively via fitting a psychometric model (i.e. the reduced reparameterised unified model) to empirical data (i.e. the checklist marks) through the computer program "Arpeggio Suite." The final Q-matrix was found to be valid and useful; it had far fewer parameters but greater power to discriminate masters and non-masters of academic writing skills. The study found that the cognitive diagnostic model (CDM)-based skill diagnosis could identify the strengths and weaknesses in the five writing subskills for students across three proficiency levels and could provide richer and finer information than the traditional raw-score approach. However, limitations and caveats of the CDM approach were also observed, which warrant future investigations of its application in assessing writing.