With the aim of improving the critical current density (J c) in theMgB 2 superconductor, minor Cu (3 at%) was doped to the MgB 2 samples in-situ sintered with Mg powder and sugar-coated amorphous B powder. Combined with thermal analysis, phase identification, microstructure observation and J c measurement, the effect of minor Cu addition on the sintering mechanism, microstructure and critical current density of sugar-doped MgB 2 superconductors were investigated. It is found that the minor Cu addition could obviously accelerated the MgB 2 phase formation and improve the growth ofMgB 2 grains during the sintering process of sugar-doped MgB 2 due to the appearance of Mg-Cu liquid at low sintering temperature. On the other hand, the Mg-Cu liquid hindered the reactive C released from sugar entering in the MgB 2 crystal lattice. Hence, the connectivity between MgB 2 grains was improved accompanying with the C substitution for B is decreased. At 20 K, the J c of codoped samples at low fields was further increased whereas it is decreased at high fields, compared with the only sugardoped samples. © Springer Science+Business Media, LLC 2012.