Event related potentials (ERPs) were used to examine the processing of literal idioms and metaphoric idioms. We compared the patterns of brain electrical activity elicited by processing four-character expressions denoting literal, conventional metaphoric meaning. Participants performed a yes-no judgment task in which they decided whether each idiom conveyed a metaphoric meaning. N400 amplitude after the onset of each idiom were recorded and analyzed. The results show that metaphoric idioms do not necessarily elicit right-hemispheric activation which has been controversial in recent years. However, Conventional metaphor and literal idioms have different brain activity which is consistent with Giora's 'Graded Salience Hypothesis'. Cz electrode's more negative response with metaphor seems to suggest that Lakoff's metaphorical mapping theory has neural correlates. These findings are consistent with recent brain imaging studies and complement them by adding the spatial analysis capacity involvement.