This study aims to reveal last year secondary school students’ conceptual understanding of image and image formation by plane mirrors before and after traditional teaching. Case study method, which is one of the survey models, is utilized in this study and the sample consists of randomly selected 147 students from six secondary schools in Balıkesir. A conceptual understanding test involving open-ended questions was designed by the researcher and administered before and after teaching and semi-structured interviews were conducted with four students during the data collection process. It was found that students confused the image formation with shadow and illumination phenomena. Moreover, students had difficulty drawing the image of the object placed in front of a plane mirror and proposed interesting ideas about differentiating real and virtual images during the interviews. Implications concerning teaching of the image formation by plane mirrors were drawn.