The purpose of this study is to verify the mediating effect of interaction among learners in a Web Based Collaboration Learning (WBCL) environment. 254 Korean college students served as test subjects and during the 4 weeks of research period, they studied the Test of English for International Communication (TOEIC) in a web-based collaborative learning system. The interaction between learners was looked into by categorizing the concept into task oriented information sharing activities and relationship oriented communication activities and analyzing the causal relationship between the two activities. Learning performances were measured in individual level. The results are as follows. First, task oriented information sharing activities effect positively on relationship oriented information sharing activities. Second, the managerial characteristics of WBCL had a positive effect on interaction between learners but the systematic characteristics had partial influence on interaction between learners. Third, the interaction between learners completely interconnects the managerial characteristics of WBCL and learning performance but partially interconnects the systematic characteristic of WBCL and learning performance. In conclusion, this study implies that managerial and systematic characteristics of WBCL should be considered on the preferential basis for the WBCL to become successful and interactive activities such as information sharing and communication should be encouraged to be active from a small-size WBCL perspective.