This study aimed to investigate the effects of cognitive diagnosis model (CDM)-based teacher feedback on student writing ability and their perceptions. CDMs have been applied to various tests of language proficiency due to their ability to provide fine-grained information about a student's language proficiency. However, it has rarely been studied how this information can be incorporated into language instruction. Therefore, this study administered a multiple-choice writing exam to Korean English learners, analyzed the test results using a CDM, and used this information to evaluate student writing ability and provide feedback on their writing. The effects were measured in three ways: a comparison of the CDM analyses conducted at the beginning and end of the semester, a qualitative analysis of two focal students' writing, and an analysis of student questionnaires. The results revealed that most students increased their mastery level at the end of the semester. Also, the CDM-based feedback brought positive changes to the writing of the focal students. Finally, students had positive perceptions of the CDMbased feedback. These findings imply that CDM-based feedback can contribute to English as a Foreign Language (EFL) students' improvement in their writing abilities by adjusting it to their needs.