The effectiveness of teaching academic courses such as mathematics and science in a foreign language has been investigated by several international studies in the literature. Even though the studies conducted have brought up contradictory results, most of them reveal that learning academic courses through a foreign language medium may pose conceptual, linguistic and psychological problems. Until now no research investigating the effect of foreign language on conceptual understanding has been conducted in Turkey. In this study, the effect of teaching in a foreign language on students' understanding the concept of Energy in a science course was investigated. Causal-comparative research design was used to determine the differences between students who took the science course in native and foreign language and the effect of language on conceptual understanding. The results indicated that students who were taught "Energy" in a foreign language, English, had more misconceptions than the students who were taught in their native language, Turkish.