This study investigates the effectiveness of applying Readers' Theater as remedial instruction for underachieving students. The participants of the study are 49 underachieving Freshman English students. The experimental group was taught using Readers' Theater as remedial instruction, and the control group received regular remedial instruction. The purpose of the study is to assess the effectiveness of instructing underachieving English learners using Readers' Theater by measuring changes in their speaking and reading abilities, English learning outcomes, and English learning attitudes. Qualitative data was obtained by administering pre- and post-tests gauging oral reading fluency, English learning achievement, English self-efficacy, English learning anxiety, and English learning motivation as well as collecting interview data to gain a deeper understanding of student reactions to the use of Readers' Theater. The results of the study indicate that the experimental group performed significantly better than their control group counterparts in terms of oral reading fluency, English learning achievement, English self-efficacy and English learning motivation, but did not display significantly lower English learning anxiety. Furthermore, the incorporation of diverse teaching strategies in a comfortable and supportive learning environment facilitated the successful application of Readers' Theater.