We examined the effect of gender and socio-economic status of Turkish 8th graders on their conceptual knowledge, scientific reasoning, and nature of science (NOS) understanding. Results showed females performed better than males on physics conceptual knowledge test. However no difference between males and females was found for scientific reasoning and NOS understanding. Besides high SES students performed better than low SES students on scientific reasoning and NOS test. No physics conceptual knowledge difference was found between low and high SES students. Implications for science education were discussed according to these findings. (C) 2015 The Authors. Published by Elsevier Ltd.