This study investigated mental rotation for identical stimuli and mirrored stimuli by both behavior response and event-related desynchronization (ERD) of EEG signals. Results showed that subjects had longer response time for mirrored stimuli than identical stimuli. Beta-band desynchronization appeared in whole brain with the parietal-occipital dominance. The ERD in beta band recovered slowly in an angular order after 450-600 ms of stimulus onset except the sharp rebound in the case of identical stimuli with rotation at 0 °. This temporal difference of beta ERD between the identical and the mirrored stimuli at 0° rotation and the ERD topographic difference in left fronto-parietal regions, together with the behavior difference, may imply an extra flip process in mirrored condition.