We investigated the structure and magnetic properties of Co2 MnSi/MgO/n-Si (100) films with the goal of achieving efficient spin injection. The Co2 MnSi films were fabricated by dc sputtering and post annealed at 400 °C for 1 h. They were oriented along the 〈 100 〉 direction with a MgO barrier layer. A strong chemical reaction was observed between the Co2 MnSi thin films and n -Si substrates in the absence of the MgO barrier. The diffusion or chemical reaction may occur up to a MgO layer thickness of 2 nm. With a MgO layer that is 2 nm thick, Co2 MnSi possessed large saturation magnetization and low surface roughness at room temperature. The electrical (I-V) characteristics of Co2 MnSi/MgO (2 nm) /n-Si (100) obtained at various junction sizes were symmetric, suggesting that MgO was an effective tunnel barrier.