The human central nervous system integrates different sensory modalities with the visual information to produce a coherent mental representation of our own body, making us capable not only to process sensory events but also to plan and executes movements in the surrounding space. The basis of Mirror Therapy (MT) is the use of a mirror to create a visual reflection of an affected limb to create an illusion of movement of the paretic part of the limb. One of the uses of the MT is in motor recovery in post-stroke hemiparesis and even thought that it is valuable rehabilitation tool, its underlying neurophysiological manifestations and interaction with Motor Imagery (MI) are still unknown. Our study aim is to assess the effect of the MT by applying Forearm Bisection Test (FBT) and EEG measurement of the Event-related (de)synchronisation (ERD/S). Our results show that FBT scores were significantly higher in experimental Mirror Box (MB) group compared to control (CN) group (median 13.54 vs 0.00, respectively; p = 0.003). Furthermore, ΔERD/S (post-pre) differed significantly between the hemispheres in the MB group in the Mu, betalow and betahigh EEG bands, whilst did not differ in the CN group in the Mu and betalow bands. The results demonstrate improvement in ERD/S MI and an update of the body representation caused by MT. Moreover, findings suggest that the reflection of the moving hand in the mirror created an illusion of concomitant movement in the opposite hand that modulate the arm length representation which is detectable even during MI at EEG level. Our findings of neural basis and link of the MT and MI supports MT as favourable neurorehabilitation tool for motor recovery affecting not only the areas governing the moving hand but also the corresponding regions of the other hemisphere.