Concept of energy is of significance in both daily life and the school curriculum, yet misconceptions about this phenomenon are frequent. To assess the extent of such misconceptions among male sixth graders, a cluster sample of 136 of them from two districts of Tehran was selected and then given a test consisting of nine open-ended questions. Additionally semi-structured interviews were also conducted. The data analysis revealed many misconceptions about this phenomenon including sources of energy, conservation and transformation of energy, and the concept of work. In addition, students’ learning in the previous grades has not prevented the development of such misconceptions. Hence, it is necessary that more attention be paid to this concept in re-designing the primary school curriculum.