The aim of this study was to develop a model of students' energy concept development. Applying Case's (1985, 1992) structural theory of cognitive development, we hypothesized that students' concept of energy undergoes a series of transitions, corresponding to systematic increases in working memory capacity. The US national sample from the Third International Mathematics and Science Study (TIMSS) database was used to test our hypothesis. Items relevant to the energy concept in the TIMSS test booklets for three populations were identified. Item difficulty from Rasch modeling was used to test the hypothesized developmental sequence, and percentage of students' correct responses was used to test the correspondence between students' age/grade level and level of the energy concepts. The analysis supported our hypothesized sequence of energy concept development and suggested mixed effects of maturation and schooling on energy concept development. Further, the results suggest that curriculum and instruction design take into consideration the developmental progression of students' concept of energy.