This research was concerned with the impact of code-switching on teaching and learning effectiveness. It aimed to find out the types and functions of code-switching used by teachers as well as to examine the impact of code-switching on learning effectiveness. The data found based on the case study of two biology teachers at SMP Fatih Bilingual School. In collecting the data, an observation checklist and semi-structured interviewwere used. This research showed that biology teachers used code-switching in four different types; tag-switching, inter-sentential switching, intra-sentential switching, and intra-word switching. The teachers also used code-switching for affective function and repetitive function. Furthermore, there are several positive impacts of code-switching on teaching and learning effectiveness in a bilingual class, including content understanding, familiarity with the terms of both languages used, classroom management, and achieved learning objectives. The finding indicated that code-switching is good to be implemented in teaching a bilingual class.