We present a detailed analysis of the effects of excess Mg on the lattice parameters, the critical temperature (Tc), the critical current (Jc), the irreversibility field (Hirr), and the upper critical field (Hc2) of MgB2 bulks and wires produced by an in situ solid-state reaction method and the powder-in-tube method, respectively. It was found that Jc, Hirr, and Hc2 were significantly enhanced for Mg excess samples. All these properties were highly sensitive to the processing temperature for Mg excess samples, while there was only a weak dependence on processing temperature for normal ones. The Tc variation for the 10% Mg excess sample was 1.6 K (36.337.9 K) when the sintering temperature changed from 650 to 850 ÃÂ°C while it only varied by 0.5 K (37.237.7 K) for the normal sample. The low-field Jc for the 10% Mg excess samples sintered at 750 ÃÂ°C increased by a factor of 3, compared to that for the normal MgB2 samples, while the Hc2 for the 10% Mg excess samples sintered at 650 ÃÂ°C reached 8.7 T at 25 K, compared to 6.6 T for the normal sample. Rietveld refinement x-ray diffraction (XRD) analysis showed that the MgO content was reduced in the 10% excess Mg samples, leading to an increase in the effective cross section of the superconductor.