EEG mu rhythm is one of the prominent oscillatory patterns for investigating the dynamics of the human mirror system. However, because of the blurring effect caused by the volume conduction in EEG measures, the origin of this rhythm remains an open question. Here we propose a novel method to identify the EEG distribution on the scalp by using the simultaneous fMRI and EEG recording. The results indicate that the mu rhythm appeared on the lateral central sites indexes as activities in the sensorimotor cortex, while the contamination from activities in other cortices also appears as the mu rhythm.