We have studied 73 pre-service science teachers' conceptions about the nature of science (NOS) using 120 true-false test items based on AAAS Benchmarks statements. We have found that participants have inadequate understanding of the NOS, especially in understanding of five categories of conceptions; change and continuity in science, bias in scientific investigations, hypothesis in scientific investigations, things common in science, and science ethics. The result also indicates that there is a difference between primary pre-service teachers and secondary pre-service teachers. From the analysis of consistency, we also found that this inadequate understanding comes from confusion rather than misconception. All the results support that there is a need of intense pre-service teacher education concerning thenature of science.