The purpose of this study is to investigate the factors of volunteering to speak in class for college students and the results will be used to improve teacher-student interaction in the college classrooms. Semi-structured interviews were conducted with 27 college students to explore the factors. Open coding approach was used to analyze the qualitative data. The results indicated the prerequisite of volunteering to speak for college students lies in the possibility of producing answers or questions. If the prerequisite was met, the college students' speaking behaviors were to be interactively determined by three inner perception factors: students' personality or confidence, their expectation of teacher's feedback, and the concern about peer reactions. Furthermore, most of the inner perception factors were affected by personal experiences and socio-cultural contexts. According to the results, this study proposed a model on volunteering to speak in the college classrooms. It is recommended that teachers encourage student to speak voluntarily by removing some barriers or doubts in class. For further study, questionnaire methods and teaching experiments are recommended to test the model.