Relaxation property is always matter of interest for the frequency dependent study of the ferroelectric materials. The Nyquist plot of the impedance data provides the information about the relaxation phenomena. Often, in solids, instead of a single relaxation time a distribution of the relaxation times is found, which arises from the different mechanisms. Usually, the relaxation times is varied by substitution and its influence is studied on the basis of the compositional effects, but the impact of the size of the substituent is rarely analyzed. Fundamentally, the substituted cations modulatethe crystal structure of the parent material, which attunes the relaxation phenomenon. In the current study, the Rb substituted lead free Na0.5Bi0.5TiO3 (perovskite structure, ABO3 type) is chosen as its A-site is compositionally disorder due to the presence of the different cations with dissimilar radii. The samples are prepared through the solid state sintering method. The imaginary part of the complex impedance (Z"") vs its real part (Z') are plotted for all samples to verify the response of the grain and grain boundaries. At the higher temperatures (=500°C) single semi-circular arcs are formed by the materials for all the compositions. Interestingly, two semi-circular arcs are observed for the material for the higher cation-site disordered system at lower temperature (below 300°C). The second circle is associated with the different relaxation time of the dipolar entity, which may be generated due to the substitution of the Rb1+. This observation prove that the cation size mismatch creates the variation in the relaxation times at the lower temperatures. The cation size mismatch also modulates the contribution of the grain boundary to the impedance property. The real part of impedance of the 4 mol %, substituted Na0.5Bi0.5TiO3 is enhanced than parent Na0.5Bi0.5TiO3, which fortifies its performance as a capacitor and energy storage device. © 2020 American Institute of Physics Inc.. All rights reserved.