The pseudobinary series Fe x Mn 1 - x Si shows a transition from an insulator for x=1 to a metallic state with long range magnetic order at the Mn-rich side. In order to understand the mechanism of the magnetic phase transition, we perform ab initio calculations for the electronic band structure and the magnetic properties of Fe x Mn 1 - x Si. The calculations are based on density-functional theory with the local-spin-density approximation. The pressure effect on the magnetic properties of MnSi are calculated. It is found that the magnetic phase transition can be attributed to the hybridization among Mn 3d, Fe 3d and Si 3p orbitals.