NdBi₄Ti₃FeO₁₅ and Bi₅Ti₃FeO₁₅ ceramics were prepared by solid state reaction. The effect of Nd³⁺ on dielectric and electrical properties was investigated in a wide range of frequencies and temperatures by dielectric/impedance spectroscopies. The combination of impedance and electric modulus analysis enabled the distinguishing of two relaxation behaviors that were assigned to originate from grains and grain boundaries. Kinetic analysis of temperature-dependent dielectric data was performed to probe the defect-related conduction. The results demonstrated that the Nd³⁺ substitution for Bi³⁺ significantly suppressed the conductivity and the ferroelectric transition temperature was decreased to ∼250 °C in NdBi₄Ti₃FeO₁₅.