The evolution of language use followed by the trend of mixing two or more languages creates an interesting phenomenon for further discussion and research. the mixing of English and Indonesian codes is currently happening in our society today. Inevitably also in schools as the main place where the transfer of knowledge occurs. The aims of the study were to analyze the types of code-mixing that used by the teacher in EFL class at SMPN 16 Semarang, to describe the teacher’s reasons for using language code-mixing in the teaching and learning process. The researcher applied descriptive qualitative research. The data was collected through observation and interviews. The participants of this study was the 9th-grade students in the EFL class. The analysis of the data showed that the teacher that there were four types of code-mixing that were used and the reason teachers teach by using a mixture of two language codes is to help students understand the material and make it easier for students to receive instructions from the teacher.