In present study, it was aimed to determine the effects of traditional teaching on levels of conceptual understanding of prospective science teachers on protein and protein synthesis before, after and six months after of instruction. Firstly, according to the views of the expert in the area, concept analysis was carried out about protein and protein synthesis. Considering the concept analysis, a six-item conceptual understanding test was prepared and administered as the pre-test, post-test and delayed post-test. As a result of the study, it was determined that the prospective science teachers had some difficulties in understanding concepts about protein and protein synthesis and traditional instruction was insufficient to overcome these problems. Especially, it was revealed that the candidates had severe misconceptions about process of protein synthesis and structure of protein. Finally, some suggestions were presented with the support of the findings obtained from this study.