Many studies report positive learning experience and improved performance in online collaborative learning. However, such learning can also incur unnecessary or excessive stress with a resultant adverse effect on the learning. This study aimed to determine the stress factors in online collaborative learning as perceived by 226 Japanese university students using English in their online interactions, and to investigate the relationship between the learners' perceptions and these stress factors. It identified four key factors affecting stress in online collaboration: "Self-efficacy," "Instructional Design," "Technology Use" and "Collaborative Process." The implications of this finding for policy-making and practice are discussed and further research in this field is suggested. (Contains 4 tables.)