In the context of the growing global concern about Education for Sustainable Development (ESD) and the popularity of forest activities in Swedish preschools, this study interviewed preschool teachers about their understanding of ESD and their use of forest activities for ESD. After analyzing the interview data based on the conceptual theory of ESD and Dewey’s theories of learning, it was concluded that most of the teachers’ understanding of ESD was incomplete and limited to the environmental aspect. It was also found that teachers followed children’s interests, and used forest activities to enable children to learn how to protect the environment through experience and practice, so as to achieve ESD. This study argues that forest activities contribute to ESD. However, the teachers’ limited knowledge of ESD and natural science in the current study restricts the practical activities of teaching ESD. Therefore, this study recommends strengthening the theoretical knowledge of ESD and natural science of preschool teachers with in-service training.