Idioms have attracted great attention over recent decades. Nevertheless, controversies still remain in regard of the most plausible cognition mechanism for idiom comprehension, especially concerning non-Indo-European languages and second/foreign language users. Event Related Potential （ERP） experiments on proficient English learners whose native language was Mandarin Chinese were conducted to test the influences of idioms＇ familiarity on both Chinese and English （as foreign language） idiom comprehension. Data including accuracy rates, reaction time, and latency and peak amplitudes of N400 were recorded and analyzed, indicating that familiarity might play a significant role in idiom comprehension in both native and foreign languages. Further, idioms, should they have similar familiarity levels, may be easier to process in the native language （Chinese） than in the foreign language （English）.
