Many researchers have reported that there is a significant gap between theory and practice in education. This research sought to contribute to this work by examining the theory/practice gap in secondary school science teaching in South Korea. To do this, a questionnaire was developed to investigate the gap between Korean science teachers' knowledge about Educational Theories and Teaching Strategies (ETTS) and the usage of it in their science classroom. The questionnaire was administered to 87 science teachers and results showed that even though participants were knowledgeable about many ETTS, only 26% of the teachers reported using it in their teaching. Major reasons reported for this gap in theory and practice were restrictive educational environments that did not support the use of ETTS, irrelevancy and difficulties of ETTS, and students' low interest in learning science. However, teachers' perception of the importance of ETTS positively affected their usage of ETTS. Implications of the results are discussed, and alternative in-service training program is suggested to activate science teachers' ETTS what they already know and to guide them to use ETTS in their actual science teaching