Rubrics are often used as scoring tools in evaluating student writing performance. Although there is a considerable amount of research on the use of rubrics in ESL writing instruction, little attention has been paid to studies of engaging students in rubric co-construction in the ELT context. This study investigates the implementation of teacher-student rubric co-construction as an alternative formative assessment in an ESL university writing course. Through analyzing the data collected from student interviews and teacher reflection, this study focuses on elucidating the rubric co-construction process and providing insights of students and the teacher who have engaged in rubric co-construction in ESL writing teaching and learning.