Abstract: Bilingual researchers found that language switching cost existed commonly in language switching. This study employed picture naming paradigm to investigate the language switching cost and its difference in Chinese-English bilinguals of different proficiency. The results showed that there was language switching cost in Chinese-English bilinguals; the less proficient bilinguals demonstrated switching cost asymmetry while the highly proficient bilinguals did not. This suggests that the proficiency of the second language is one of the important factors affecting switching cost asymmetry. It supports the language specific selection threshold hypothesis.