In recent years, there has been growing interest in exploring argumentation about socio-scientific issues (SSI) in the classroom to improve students' scientific literacy. Thus, this research aims to investigate how intervention based on argumentation about SSI affects secondary students' patterns of informal reasoning and reasoning quality. The action research was conducted with 16 secondary students in which all of them were given pre-test and post-tests. The collected data were analyzed qualitatively and quantitatively. The results suggest that participants frequently relied on rationalistic informal reasoning or integrated informal reasoning patterns to solve SSI. Students' reasoning quality also improved as there were higher frequencies of students with a higher level of reasoning quality and a significant increase in the construction of supportive argument, counterargument, rebuttal, and the total number of arguments in the two post-tests when compared to the pre-test. This study provided clear support for the potential of argumentation to improve secondary students' scientific literacy by promoting the construction of evidence-based arguments to assist in making rational decisions when solving SSI.