MgB2 /Fe wires with 10 at. % excess Mg produced by in situ powder-in-tube processing werecompared to normal stoichiometric MgB2 /Fe wires prepared by the same method. It was found thatthe critical current density Jc and the irreversibility field (Hirr) were significantly enhanced forMgB2 /Fe wires with excess Mg. The transport Jc for 10 at. % Mg excess samples sintered at 800 degrees C, measured at a field of up to 14 T, increased by a factor of 2 compared to that for the normal MgB2 wires. The best Jc results for the 10 at. % Mg excess sample were obtained by heating thesample at 600 degrees C for 1 h; the Jc at a field of 8 T and at temperature of 10 K reached 3104 A/cm2. The detailed analysis of the effect of excess Mg on the microstructures, the Jc, andthe Hirr, of MgB2 /Fe wires are presented in this paper.