In order to understand the penetration process of a solar wind filament into the magnetosphere in detail, a simplified two-dimensional PPMMHD code for ideal MHD is applied to the penetration. The filament is modeled as a field-aligned cylinder impinging on the dayside magnetosphere. The initial magnetopause is considered as a purely tangential discontinuity without width. The results in this paper show that a plasma filament in the magnetosheath with excess momentum density will penetrate through the magnetopause and become irregularity inside in the magnetosphere. This paper presents detailed pictures for the distribution and the evolution of the plasma mass density, thermal pressure, flow velocity, and magnetic field around the filament. The results in this paper confirm our expectation that the penetration in the case with antiparallel magnetic fields happens in the same way as in the case with parallel magnetic fields in the initial situations both without and with a diamagnetic current flowing on the surface of a filament under the approximation of two-dimensional ideal MHD. ¿ American Geophysical Union 1994