Being bilingual provides opportunity to choose the code used in interaction. Codeswitching (CS) is also the result of bilingualism and contact among languages. This study attempts to discover the types of CS (English-Bahasa Indonesia) and how the function of code-switching is in lecturer-student's interaction in thesis examination. This study employed qualitative content analysis as the research design. The data were in the form of utterances obtained from recorded interaction of two thesis examination sessions in the graduate program of English at Universitas Sumatera Utara, Indonesia. The data were analysed through some processes including data condensation, data display (identifying, labelling, grouping), and inference making. The utterances were analysed by using Poplack's classification of CS. From the analysis, it was found that there are three types of CS namely intra-sentential, intersentential, and tag switching. Then, the dominant type is intra-sentential switching. The functions of CS utilized by the participants in the interaction are as entertaining, translating, emphasizing, making an inference, clarifying, explaining, asking, checking understanding, giving feedback, giving discussion, and repetition. Lecturers and students in the interaction utilized CS in order to keep the continuity and smoothness of the interaction as well as to ensure that the messages could be comprehended well.