Abstarct: Magnetotelluric (MT) method along with seismic method is one of the most important methods that is used for identification of complex structures. For this reason, MT data acquired along survey line A were selected and interpreted to recognize the hydrocarbon structures in Sarab area, Gachsaran. First, static shift correction was carried out on the MT data using time domain electromagnetic data. Then, dimensionality analysis was made on the MT data, and it was concluded that the earth in the area was of three dimensional (3D) and two dimensional (2D) natures. In the next stage, one dimensional (1D) and 2D modeling of the MT data using the Occam’s algorithm was made with the help of ZondMT2D and WinGlixnk software packages. As the earth was mainly 3D, thus 1D modeling did not contain specific information but 2D modeling resulted in good information about the subsurface structures of the area so that the Sarab and Jafarabad anticlines, located at 5 and 1.5 km from the beginning of the MT survey line and in the depth of 1 km and 750 m, were respectively detected. Furthermore, the Mishan and Aghajari formations, having resistivity contrast with their surrounding structures, were detected in top of Sarab anticline. Top of Asmari formation, forming reservoir rock in the area and having resistivity of 70 ohm-m, was also identified considering its resistivity. A deep faulting, occurred in the end of the survey line, was also detected. The objective of this research is to determine dimensions of subsurface structures in an oil field, located in south west of Iran. By using parameters such as normalized weighted index, ellipticity, and Wall's rotational invariant measure, this goal can be achieved. Using ellipticity factor in frequency range of 1-320 Hz, the earth is represented in 2D from, however, in lower frequencies, the earth should be represented by 3D form. In most MT stations, normalized weighted index has indicated that the earth is in 2D form on the surface or shallow subsurface, but it is represented by 3D shape in higher depths. In this regard, Wall's rotational invariant measure has shown more heterogeneity. This measure has indicated that the earth is in 2D & 3D form on the surface or shallow subsurface and is perfectly 3D in higher depths, but the earth dimension cannot be determined in some certain frequency ranges. It is also found out that the earth in both shallow and deep parts of the study area indicates high heterogeneity.