The alternative conceptions of secondary school students on energy source for living organisms have beets studied by questionnaire. For this study, total 298 students in grade 8-12 were sampled from secondary schools in seoul. The results showed that many students understood energy source scientifically. But, some alternative conceptions occurred in that hon-energy source such as water, oxygen, carbon dioxide were thought to supply energy. The proportion of alternative conception reduced with grade. The students who had alternative conceptions might think that energy source was not so much energy supplier as requirements for living or energy itself.