This study surveys the experience of pre-service Korean language teachers who integrated their subject area with other subject areas in their classes, and its aim is to examine the teachers’ perceptions of this experience and the significance it had for them. For this purpose, the present study analyzed the teachers’ self-reflection journals recorded at three different points in their experience, small group interviews, and teacher efficacy measurement data. The analysis revealed that key features of the experience noted by the pre-service teachers can be summed up in terms of the ‘difficulty’ they had in designing subject-integrated classes, the ‘psychological changes’ they underwent during this process, and their resultant ‘understanding’ of subject integration. More specifically, ‘difficulty in condensing diverse subject matters into one’ and ‘difficulty in formulating the details’ were salient features of their experience. Despite such ‘difficulty,’ however, the teachers experienced a shift in their psychological attitude from ‘discouraged’ to ‘beneficial,’ ‘interesting,’ ‘confident,’ and ‘hopeful.’ Lastly, it was found that, as a result of these experiences, the pre-service teachers attained a clear grasp of the concept of subject integration, and expanded their understanding of subject-integrated classes. Moreover, measurements of the teacher efficacy of these pre-service teachers showed that there was a statistically significant increase in their teacher efficacy.