This study examined Taiwanese L2 university students' critical thinking dispositions (CTDs) in an online writing environment. The researcher developed an educational game called MOSES with cooperating blogs to motivate students and improve their writing abilities through online collaboration. Students completed a questionnaire to assess their CTDs based on Facione's framework of seven sub-dispositions. The findings revealed that the students displayed open-mindedness, systematicity and inquisitiveness, but low interaction and motivation. The study provided initial insights into L2 students' CTDs and how online collaborative environments may help develop their critical thinking skills.