The aim of this study was to address the influence of implementing English-only instruction to promote the English speaking proficiency of rural junior high school students. This study employed both quantitative and qualitative approaches by way of action research to collect and analyze the data. Twenty-three eighth grade students of Le-Le High School in Pingtung County were invited to participate in the present study. They participated in the English-only instructions for ten weeks with two periods of class per week. The ten-week English-only activities were mainly designed according to the content of the English textbook, published by Kang Hsuan Educational Publishing Group, as well as Let’s Talk in English magazine published in September, 2018. The English speaking proficiency of the students was examined through a test designed by the foreign instructor and the researcher. Furthermore, the class observation logs, interviews, and students’ learning feedback were adopted to analyze the implementation of English-only instruction to promote junior high school students’ English speaking proficiency. Results indicated that in order to achieve the teaching goals, it was better to implement English-only instruction to go with the English-only teaching plan which meets students’ needs. Moreover, the English-only instruction required a lot of communicative activities to promote students’ English speaking proficiency while implementing English-only instruction. Additionally, the design of differentiated instruction was needed to achieve the instruction goals while implementing English-only instruction. As a result, the students’ English speaking proficiency was enhanced by way of English-only instruction. Based on the conclusions, this study offered specific recommendations for future implementations of studies of English-only instruction.