This study investigated a phenomenon of bilingualism in which the use of the target language (English) is switched to the mother language (Vietnamese), known as code-switching. The purpose of the study was two-fold: (1) to describe the functions for which EFL teachers’ CS served in the General English classes with pre-intermediate proficiency level at a tertiary institution, and (2) to measure the teachers’ attitudes towards using CS in teaching EFL. A mixed-method research design was adopted using a questionnaire and stimulated-recall interviews. English teachers (n=45) responded to the five-Likert scale questionnaire and stimulated-recall interviews were held with 5 teachers teaching at a tertiary institution. Quantitative data from the Likert scale questionnaire items were analyzed using descriptive statistics to reveal frequency, mean and standard deviation scores, an inductive content analysis was applied for the analysis of qualitative data. The findings revealed that the teachers used CS for pedagogical, administrative, and interactional purposes. The results of the study also indicated that the majority of the teachers code-switched to Vietnamese to accommodate low proficiency students and explain difficult concepts such as grammar and vocabulary to the students. From the findings, suggestions were put forward on how to use code-switching effectively.