This study investigated phase transitions in EFL students' speech fluency development using a complex dynamic systems perspective. Two students with different proficiency levels were selected from an intact speaking class. These students learned and practiced specific strategies to improve their speech fluency. Phase transitions were analyzed based on three criteria: sudden jumps, anomalous variance, and qualitative change in the attractor. Number of syllables was used as the speech fluency measure. The results suggest that only the higher-proficiency student underwent one phase transition. These findings imply that a short explicit fluency strategy training intervention could improve the speech fluency development of high-proficiency students in EFL classrooms. The results have implications for theory and pedagogical practice relating to EFL students' speech fluency development.