High-temperature superconductors with a nominal composition Bi2Pb0.3Sr2Ca2Cu3-xCoxO10+δ for (0≤x≤3) were prepared by solid-state reaction method. Effects of the Co substitution on Cu sites have been investigated to obtain the optimum concentration for the formation and stabilization of the superconducting samples. The results of electrical resistivity measurements of the samples showed that increasing x substituting Co enhance the superconducting properties except for the samples with x=3 yield semiconductor behavior. The higher critical temperature TC found at 118 K, which is for the composition Bi2Pb0.3Sr2Ca2Cu2.2Co0.8O0+δ, which has the highest value of excess oxygen content 0.315. The X-ray diffraction analysis for all superconducting samples showed an orthorhombic structure with two phases, high-Tc 2223 phase and low-TC 2212 phase. X-ray spectroscopy analysis showed that the compositions with x=0.6 and 0.8 samples did not contaminate during the synthesis processes. The scanning electron microscope has been used to identify the morphology of the superconducting phase. The plate-like grains of the high Bi- 2223 phase appeared in most micrographs of the superconducting samples besides changes morphology of samples with increasing dopant concentration.