This study aimed to investigate the effects of on-line argumentation on 37 university students' informal reasoning regarding a socio-scientific. In addition, such effects on students with different reasoning abilities were also explored. The students were asked to discuss the issue, xenotransplantation, anonymously in the on-line discussion forum in groups for a week (7 days). This study revealed significant effects of on-line discussion task on improving the students' informal reasoning quality. More importantly, it was found that both the students achieving a higher reasoning level and those achieving a lower reasoning level benefited from the anonymous on-line discussion, but in different ways. Both the students in the two groups proposed significantly more arguments after on-line discussion task; but only the students achieving a lower reasoning level performed significantly better in their rebuttal construction and usage of different reasoning modes after the on-line discussion task.