This study investigated the relationship between pre-service science teachers' written argumentation levels about socio-scientific issues and epistemic belief levels in an online discussion environment. A mixed-methods approach was used: 30 Turkish pre-service science teachers contributed with their written argumentations to four socio-scientific issues. The pre-service science teachers' argumentations were evaluated by an adapted version of argumentation analysis framework developed by Sadler and Fowler (2006) and their epistemic belief levels were measured using the Epistemic Belief Questionnaire by Kuhn, Cheney, and Weinstock (2000). The qualitative as well as quantitative results indicated that: (1) the pre-service science teachers produced high-level argumentations for each socio-scientific issue in an online discussion environment, (2) levels of argumentations increased from climate change to human genome project issue, and (3) higher-level argumentations were produced for higher epistemic belief levels (i.e., multiplist and evaluativist).