The direct-current (dc) electric-field dependence of dielectric constant around paraelectric–ferroelectric transition in Cd2Nb2O7 has been studied. The results show significant suppression of the dielectric constant by the application of dc bias. The electric-field dependence of the dielectric constant at the temperature around paraelectric–ferroelectric phase transition can be well described by the modified Devenshire relation including a cluster term, i.e., ε(E)=ε1−ε2E2+ε3E4+(Px/ε0)[cosh(Ex)]−2. The fit parameters indicate that the polar-cluster carries polarization P=∼0.5–2.5 mC/m2 with the cluster size of L=∼11–15 nm.