Scientific epistemological beliefs reveal the philosophical understanding of the science subjects about the meaning, producing and sharing processes of scientific knowledge. It is important to use informal environments, museums, science centers, natural parks, aquariums, all kinds of natural area, during science teaching, because students interact with nature and natural phenomena by acting like scientists. Moreover, teachers can easily follow whether students use scientific processes and try to find solutions to problems by using these processes. One of the informal learning environments is Science Fairs which are public exhibitions where students expose their projects to visitors. Students develop their research skills, critical thinking skills and positive attitudes toward science during the science fair. The study was conducted to examine the changes in the scientific epistemological beliefs of adults participating in the 12 science fairs held on different days in the province of Bornova in Izmir and their views on the science fair as informal teaching environment. The sampling group consisted of 92 individuals (69 female, 23 male) selected from the people visited to science fair by random sampling method. The results were analyzed using quantitative and qualitative data collection tools. The research design is a single group pretest - posttest design. The Scientific Epistemological Beliefs Scale, consisting of 30 items of 5 Likert type, was applied to participants. Whether or not there was a statistically significant difference in the opinions before and after the Science Fair was analyzed with the SPSS package program. Then 2 people from each science fair selected randomly from 12 science fairs for interview. 4 adults changed their mind to interview. Twenty were interviewed with 4 questions that the researcher prepared and developed via 3 different expert opinions. The results were evaluated using descriptive analysis. According to the results, there was no statistically significant difference in the mean of pre-test and post-test scores of the participants. Participants have a strong positivist science perspective. There was no statistically significant change in scientific epistemological beliefs of the participants after the Science Fair. On the other hand, it has been seen that the changes in the conclusions of the postmodern science, such as the interchangeability of scientific knowledge and the subjectivity of science.