Micro crystalline materials of BiCoO₃ and Ni_0.5Bi_0.5CoO₃ have been prepared by solid state reaction technique. XRD studies of these polycrystalline materials confirmed the cubic structure with 197 I 23 space group. The substitution of nickel in place of bismuth resulted in lattice contraction. The thermoelectric properties were investigated in the temperature ranging from 300°C to 700°C. The samples showed positive Seebeck coefficient. Nickel substitution with Bismuth is found to decrease the Seebeck coefficient and thermal conductivity but increase the electrical conductivity. The figure of merit (ZT) of the material was enhanced on nickel substitution. The ZT values increased with the increase of temperature which enables its utility in high temperature thermoelectric applications.