Double B perovskite-type series in the system La_0.6Ca_0.4Mn_{1 - x}Ni_xO_3-δ with x = 0-0.6 was synthesized for the first time in air by solid state reactions at 1200 °C from simple oxides and CaCO₃. Single phase compositions with small impurities of NiO-MnO solid solutions were formed. Replacement of Mn cations with Ni cations is accompanied by strong increase in electrical conductivity of the prepared ceramics. Lower conductivities of the Ni containing compositions compared to La_0.6Ca_0.4MnO_{3 - δ} are explained by stronger polarization of the -Mn^{(α + γ)+}-O^α--Ni^{(α - γ)+}-O^α-- fragments of the -O^α--Mn^α+-O^α--Mn^α+-O^α--Mn^{(α + γ)+}-O^α--Ni^{(α - γ)+}-O^α- chains in comparison with the -O^α--Mn^α+-O^α--Mn^α+-O^α- chains without Ni-cations, because of different electronegativity values of Ni and Mn. The compositions with x = 0-0.1 and x = 0.6 are p-type and those with x = 0.2-0.5 are n-type semiconductors. Type and level of the electrical conductivity of the La_0.6Ca_0.4Mn_{1 - x}Ni_xO_{3 - δ} series are the functions of the [Mn⁴⁺]/[Mn³⁺] and [Ni³⁺]/[Ni²⁺] ratios. Mn⁴⁺ Ni⁴⁺ and Ni²⁺-cations are possible point defects which determine the p- and n-type conductivity of compositions, respectively.