This study aims to clearly diagnose EFL "students' writing strengths and weaknesses." I adapted a diagnostic framework which comprises multiple writing components associated with three main dimensions of second language writing. The essays of 304 English learners who enrolled in an English language program were double-rated by 13 experienced English teachers. The data were submitted to cognitive diagnostic assessment (CDA). The provided evidence supports the validity of 21 writing descriptors tapping into three academic writing skills: content, organization, and language. The skill mastery pattern across writing shows that language was the easiest skill to master while content was the most difficult. In mastering language, the highest mastery levels were found in using capitalization, spelling, articles, pronouns, verb tense, subject-verb agreement, singular and plural nouns, and prepositions while some sub-skills such as sophisticated or advanced vocabulary, collocation, redundant ideas or linguistic expressions generated a hierarchy of difficulty. Researchers, teachers, and language learners may benefit from using the 21-item checklist in the EFL context to assess students' writing. This method can have additional benefits for educators to know how to adapt other instruments for various purposes.