This study examines the implementation of an English-Only Policy (EOP) in a bilingual junior high school program in Thailand and investigates its impact on students' English speaking skills as well as the challenges encountered during its implementation. Adopting a mixed-method research design, the study involved 30 Grade 1 students from a bilingual class at Ban E-Lert School, Loei Province. Quantitative data were collected using a one-group pre-test and post-test design, with students' speaking performance assessed through an analytical rubric measuring fluency, pronunciation, grammatical accuracy, vocabulary mastery, and communication ability. Qualitative data were obtained from structured classroom observations conducted over a 12-week intervention period to capture students' language use, participation, and difficulties during the implementation of the policy. The quantitative findings revealed a statistically significant improvement in students' overall speaking performance after the implementation of the English-Only Policy (t(29)-10.081, p<.001), with notable gains in fluency, vocabulary use, grammatical accuracy, and speaking confidence. Qualitative findings indicated increased willingness to communicate in English and reduced reliance on the first language during classroom interaction. However, persistent challenges were identifiel. particularly in pronunciation accuracy due to first-language phonological interference. Students frequently mispronounced English words, especially final sounds and consonants, such as sugar being pronounced as cuka, which affected intelligibility. Additional challenges included limited exposure to accurate pronunciation models and speaking anxiety during spontaneous communication. Overall, the findings suggest that while the English-Only Policy effectively enhances students speaking skills in a Thai EFL context, its successful implementation requires explicit pronunciation instruction, sufficient linguistic input, and supportive pedagogical scaffolding