A metal/ceramic nanocomposite was synthesized by defect control in a selective reduction reaction of a complex solid solution. The defect type of a (Co, Mg)O solid solution was controlled by doping oxides with different valencies. The controlled defects strongly affected the reduction reaction behaviors as well as material morphologies after the reaction. In the case of scandium doping, the diffusion was rate controlling in the reduction reaction, and metals of a nanocolumn-like structure were highly dispersed into the matrix. On the other hand, in the case of lithium doping, the interface reaction was rate controlling in the reaction, and metal nanoparticles were highly dispersed into the matrix.