Abstarct: Magnetotelluric (MT) method is a passive surface geophysical technique so that uses real passive electromagnetic surface fields for analysis of subsurface electric resistivity structures. Investigation depth in MT method is much more than the other electromagnetic ones. From 1970 to 1980, MT method was applying as an initial tool for identifying variation of thickness in large sedimentary areas. As in most cases, real studied areas inherently has complexities so 1D or even 2D modelling is not adequate for exploration targets and requirement of 3D modeling makes sense as well. In conjunction with the USArray component of EarthScope, long period magnetotelluric (MT) data are being acquired in a series of arrays across the continental US. Initial deployments in 2006 and 2007 acquired data (10–10,000 s) at 110 sites covering the US Pacific Northwest, distributed with the same nominal spacing as the USArray seismic transportable array (75 km). ModEm is a type of modular system for 3D inversion of MT data. A regular inversion program was selected for inversion of impedance four components tensor for 109 sites with acceptable good quality data for 8 periods (100 to 8000 seconds). These data were acquired from the Cascadia area. Suitable time length for calculation of 162 periods, was 12 hours while the best result was occurred in 162nd period by 1.75798 error. The most striking and robust features revealed by initial three-dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern Oregon, and beneath the Cascade Mountains, contrasting with very resistive crust in Siletzia and the Columbia Embayment. For testing results validation, residual data will be used. The results will prove the validation of inversion results.