The influence of Cu addition on the microstructure and superconducting performance of ex-situ sintered MgB 2 is systemically studied. It is found that the critical current density (J c ) of the Cu-doped sample is improved compared to the un-doped samples. In particular, the J c of the Cu-doped sample sintered at 900 °C for only 10 min is the best value at low fields. The reason is that Cu addition can obviously promote the decomposition of MgB 2 : MgB 2 ↔MgB 4 +Mg, which produces more Mg that can then react with Cu, forming local Mg-Cu liquid at high temperature. The presence of this local Mg-Cu liquid can significantly enhance the migration and self-sintering of MgB 2 , leading to the significant improvement in the grain connectivity and J c . Our results indicate that Cu addition is a promising method to fabricate high-performance ex-situ MgB 2 bulks and wires.