The focus of this study is investigating the extent to which Turkish high school students understand the conceptions of nature of science (NOS). The paper describes the use of questionnaire of NOS and a semi-structured interview. The questionnaire was applied to 162 high school students. After the questionnaire was analyzed individually, six students were interviewed. Interview transcriptions were examined individually. Based on the results, students had some deficiencies on the conceptions of NOS. Students may still hold naive ideas about what data is. Hence, science teachers should implement instruction to help the development of their students' conceptions regarding NOS. (C) 2010 Elsevier Ltd. All rights reserved.