Critical-current density (J(c)) is a parameter of primary importance for potential applications of high-temperature copper oxide superconductors. It is limited principally by the breakdown of zero-resistive current due to thermally activated flux flow at high temperatures and high magnetic fields. One promising method to overcome this limitation is to introduce efficient pinning centers into crystals that can suppress the flux flow, A marked increase in J(c) was observed in B(2)iSr(2)CaCu(2)O(8+delta) (Bi-2212) single crystals doped with a large amount of Pb. By electron microscopy, characteristic microstructures were revealed that probably underlie the observed enhancement in J(c) : thin (10 to 50 nanometers), platelike domains having a modulation-free structure appeared with spacings of 50 to 100 nanometers along the b axis.