This study investigated L2 learners' perception changes at each stage of online collaborative writing. Previous studies revealed the familiarity of L2 collaborative learning with Information and Communication Technology (ICT), whereas few described at which stage of the learning process L2 learners' perceptions change. Therefore, this study examines how the learners' attitudes and perceptions change at certain phases of collaborative learning and whether these changes affect the success or failure of their L2 collaborative learning. This study analyzed two questionnaire surveys before and after the learning activity, observed the collaborative learning processes via visualization tools, and conducted semi-structured interviews for participants to reflect on their learning processes and perceptions of collaborative writing. The mixed research analyses demonstrate that advancing a particular stage leads to the learners' linguistic awareness and the shift of their attitudes more positively. The findings show the factors and stages determining the success of L2 online collaborative learning. [For the complete volume, "Intelligent CALL, Granular Systems and Learner Data: Short Papers from EUROCALL 2022 (30th, Reykjavik, Iceland, August 17-19, 2022)," see ED624779.]