As additional preparation while teaching for the K-12 medium of instruction, the Korean Ministry of Education has actively promoted diverse professional development (PD) programs to promote English teachers' "Teaching English through English" (TETE) implementation. This study explored Korean K-12 English teachers' self-efficacy about TETE implementation and their experiences of participating in a long-term PD program (six-months). Through semi-structured interviews, classroom observations, and teachers' after-PD evaluations, the data analysis showed that PD could enhance English teachers' self-efficacy about implementing TETE, and that the PD site provided a professional learning community for English teachers to reflect on what they had learned from their training. However, further data analysis found that self-efficacy and experiences in the PD program were different for elementary and secondary English teachers. Several suggestions are provided in terms of effectively preparing K-12 English teachers for TETE implementation through the long-term PD program.