Cognitive diagnostic models (CDMs) have recently received a surge of interest in the field of second language assessment due to their promise for providing fine-grained information about strengths and weaknesses of test takers. For the same reason, the present study used the additive CDM (ACDM) as a compensatory and additive model to diagnose Iranian English as a foreign language (EFL) university students' L2 writing ability. To this end, the performance of 500 university students on a writing task was marked by four EFL teachers using the Empirically derived Descriptor-based Diagnostic (EDD) checklist. Teachers, as content experts, also specified the relationships among the checklist items and five writing sub-skills. The initial Q-matrix was empirically refined and validated by the GDINA package. Then, the resultant ratings were analyzed by the ACDM in the CDM package. The estimation of the skill profiles of the test takers showed that vocabulary use and content fulfillment are the most difficult attributes for the students. Finally, the study found that the skills diagnosis approach can provide informative and valid information about the learning status of students.