The study is aimed at exploring the types of code-switching patterns applied by 35 student teachers at a technological college, and the functions such switches served. The investigation was made in an English teacher education course offered to majors of early childhood education. The participants were night school students who worked during the day. At the end of the semester, a questionnaire consisting of twenty items and one open-ended question was distributed to collect data. The findings indicate that most of the participants switched code from English to Mandarin in situations such as "classroom management", "common command", and "question and answer sessions". The top three functions served by the code-switching in classroom interaction are as follows: "efficiency", "ease of expression", and "topic switching". The results revealed that majors in early childhood education need to acquire more instruction on English education and the use of classroom English, so that they will feel more confident in expressing their meanings in English alone. Furthermore, code-switching from English to Mandarin serves many functions in English teaching, and may play an essential role in English teaching.