Although research about the potential learning outcome of collaborative writing in L2 or foreign language education has proliferated in the last few decades, little is known about students' motivation in collaborative writing. This case study seeks to explore higher-proficiency (HP) students' motivation in L2 collaborative writing, as well as factors influencing their motivation. The findings reveal that students with similar proficiency (HP) could have different levels of motivation, which may affect their participation. This study also found that knowledge of collaborative writing, previous beliefs and experiences of pair/group work, and perceived value of the role in the partnership were three major factors that influence students' motivation in collaborative writing. This study makes several recommendations from a motivational perspective about how best to implement collaborative writing tasks in L2 classes.