Virtual Reality (VR) is becoming popular and has been used in many kind of different purposes including entertainment, daily life activities and medical field. However, with the current VR technology, people are still be able to discriminate the VR from the real world very easily. Perhaps, the better way to develop the VR technology is not by considering how much VR looks like the real world but how much similar humans perceive from VR comparing to the real world. With this aspect, we perform experiments to show that the similarity in brain signal activity can be used as the measurement of the VR quality. We record EEG signals in occipital area of the brain and calculate the similarity between two signals obtained while the subject is observing VR objects and real world object. The results show that the similarity of EEG signal increases as the VR object becomes more similar to the real world object. This result suggests that the similarity of brain signal can be used as the measurement for VR quality.