While previous research has usually discouraged teacher evaluation, such as scores, in formative assessment based on the result that it demotivates learners, these studies usually focus on end products of student learning, not taking into account the learning process. In order to provide a more comprehensive understanding of the effect of scores in formative assessment, this study investigated written products of 41 Korean learners of English and their revision. Students completed two writing assignments, including a first and second draft, receiving scores and feedback on the first draft of one writing assignment, but only feedback on the first draft of the second writing assignment. The comparison and contrast of these scored and non-scored writing assignments, revision, and perceptions reveal the following results: 1) while their scored writing assignments received higher scores in grammatical accuracy, they did not score higher in other areas; 2) students made more global level changes in scored assignments; and 3) students reported that scores motivated them to aim for higher grades, and that analytic scores in four areas (topic, organization, elaboration, and grammar) provide them with guidance for revision. These findings imply that the effects of scores in formative assessment depend on a variety of factors, such as their format and context, and that writing teachers can take advantage of scores to facilitate student learning.