Diffusivity is the quantitative measure of the relaxor nature of a ferroelectric material. The observed electrocaloric effect of the relaxor ferroelectrics due to the change of diffusivity are reported in this article. Four samples with diffusivities 1.55, 1.72, 1.94 and 2.18 are prepared by precisely doping La in Pb (Zr0.65Ti0.35)O3 [i.e. PZT (65/35)], from 6% to 9%. These values are deduced from the dielectric measurements which are performed in the frequency range from 100 Hz to 10 MHz at different temperatures ranging from 173 K to 573 K. The temperature (Tm), where the dielectric constant is the highest, is also found to decrease with increasing La concentration. For 6% doped sample, the transition occurs around 483 K and for 9%, it is around 364 K. As the value of diffusivity is varied from 1.55 to 2.18, both the values of entropy and temperature changes (ΔS and ΔT) get reduced. The highest ΔS observed is around 0.37 J kg−1 K−1 and the corresponding ΔT is 0.49 K for the diffusivity of 1.55. For the highest diffusivity, the values of ΔS and ΔT are found to be 0.12 J kg−1 K−1 and 0.11 K, respectively. Apart from these changes, it is also observed that as the transition temperature decreases with increasing diffusivity, the maximum ΔS and ΔT values occur at lower temperatures which is consistent with the change of Tm. © 2019, The Minerals, Metals & Materials Society.