Cu-Nb alloys with Nb concentration ranging from 0-30wt% were prepared by mechanical alloying (MA) at room temperature. The effects of Nb content on the crystalline refinement process and mechanical property of the immiscible Cu-Nb system have been investigated by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray detection, optical microscopy, transmission electron microscopy and microhardness measurement. It is found that the completed dissolution of Nb in Cu can be achieved in the samples with Nb content less than ~11wt% after 100 h milling, although the equilibrium solubility level is nearly zero. The grain size refinement capability of MA-ed Cu-Nb powders enhances with increasing Nb content up to 30wt%. This is because the susceptibility to recovery process becomes reduced, when the amount of Nb solutes segregated into the dislocation of Cu phase is increased. For the 100 h milled Cu-30wt%Nb powders, the average Cu grain size is only ~6nm. The microhardness of the samples shows an enhancement with increasing Nb concentration. The main strengthening mechanisms of the MA-ed Cu-Nb alloys are linked to the grain size reduction and the dissolution of Nb into Cu matrix.