This study aims to prove the effect of language proficiency on code switching by bilingual Indonesian students in learning English as a foreign language. The data were collected from 22 participants studying in English department at STKIP-MB. They used mixed Indonesian-English in the teaching and learning process. Based on English proficiency, the participants were divided to be two groups, lower and upper group. The participants were given the same tasks to read four different groups of words; Monolingual Indonesian group of words, monolingual English group of words, bilingual Indonesian-English group of words, and the bilingual English-Indonesian group of words. The statistic showed that there was no significant different time used in performing each task, it was found F(0,05)= .842, p>.05. Furthermore, the same result was found in the effect of language proficiency toward the code switching, there was no significant effect of English language proficiency on the bilingual tasks.