This research aims to examine subsurface structures in Sabalan geothermal region, northwest of Iran using threedimensional (3-D) modelling of magnetotelluric (MT) data. For this, we have used MT data of 27 stations acquired along 4 survey lines in the northwest part of the region, namely Moeil valley. These MT data have been acquired by EDC Company in 2007. Since the real earth has three dimensions, 3-D inversion has obvious advantages over twodimensional (2-D) inversion, and therefore, we have used 3-D inversion of MT data to obtain a 3-D electrical image of the subsurface structures of the region. In this research work, WSINV3DMT code has been employed for 3-D inversion of the acquired MT data. As a result, apparent resistivity and phase cross-sections have been obtained. Furthermore, dimensionality analysis of the MT data has been carried out by using polar diagrams of the impedance tensor. The results of this dimensionality analysis indicate that shallow subsurface structures can be considered as one-dimensional (1-D) or 2D structures, however, deep structures should be considered as 3-D structures. After preparing the model parameters and designing the suitable modelling blocks, 3-D Occam's inversion of the MT data has been made. The location of the geothermal reservoir in the region is determined from the results of the 3- D Occam's inversion. Moreover, the results of the 3-D inversion of the MT data have been compared with the results of the 2-D models, and then, the results have been interpreted using geological information from the region. From the interpretation results of this study, we have found that the geothermal reservoir is located at the depth of 800 to 2562 meters. ; Open-Access Online Publication: September 11, 2023