Because a cognitive diagnostic model (CDM) can provide a detailed profile of a second language learner for the purpose of diagnosing their abilities, it has been they are being used in various areas of second language acquisition and learning. However, writing has not been as extensively studied as grammar, vocabulary, or reading comprehension ability due to difficulties in scoring. To date, writing has mostly been evaluated using a direct method, that is, evaluating learners’ essays; however, this type of direct assessment has several limitations, including a degree of subjectivity and difficulties when it comes to large scale implementation. In order to compensate for the shortcomings of the direct method as well as to facilitate application of a CDM to the assessment of writing skills, this study set out to develop a multiple-choice writing test for Korean learners’ of English and use it in a real classroom setting to observe its effects on writing instruction. The results revealed that the test is valid and reliable. Additionally, when the profiles obtained from the CDM analysis of the test results were incorporated into writing instruction, they seemed to help improve the students’ writing skills. These findings suggest that the multiple-choice writing test can be used to diagnose the writing skills of learners of English and incorporated into their writing instruction.