Two kinds of Au powder having different particle size were mixed with a Bi–Pb–Sr–Ca–Cu–O superconductor, purpose being to improve the superconducting characteristics, especially
J
c. By conventional Au powder addition,
J
c appreciably increased and a maximum
J
c of 900 A/cm
2 was obtained on the Au 30 wt% mixed composite. On the contrary,
J
c decreased by mixing with ultra fine Au powder. This
J
c deterioration was predominantly ascribed to the formation of microcracks and poses by the segregation of Au ultra fine powder.