In present study, the sintering process and superconducting properties of MgB2 prepared with milled original powders of Mg, B, and 3 at.% Graphite were investigated. It was found that ball-milling treatment of original powders can significantly promote the C substitution for B in MgB2 crystal lattice, resulting in more lattice distortion. Consequently, the values of J (c) at high fields were improved. Besides, ball-milling also refined MgB2 grains, which enhances grain boundary pinning and is also contributed to the improved J (c) at high fields. On the other hand, with the milling time prolonging, Mg becomes more active and leads to an obvious increase of MgO impurities during the subsequent sintering process. These MgO impurities would degrade the connectivity between MgB2 grains and finally lead to slight decrease in J (c) at low fields.