Ba0.6Sr0.4TiO３（ＢＳＴ） and Ba(Zrx Ti1-x)O３（ＢＺＴ） (x=0.25,0.3,0.35,0.4) ceramics were pr epared by traditional ceramic processing and their structural, dielectric proper ties as well as the mechanism of its nonlinear dielectric constant under dc elec tric field were systemically examined. The results show that the phenomenologic al theory of Devonshire can be applied to BST system when the material is in the paraelectric state, and the equations εr(app)εr(0)=1［1 +αε3r(0)E2］1/3 and ε(E)=ε1 -ε2E2+ε3E4 can give a quanti tative explaination of its nonlinear dielectric behavior under high DC electric field. where εr(0) and εr(app) are the dielectric cons tants under zero DC field and under the applied field respectively, α is the an harmonic coefficient, Ｅ is the strength of electric field, ε(F) is the dielect ric constant under the applied field and ε１，ε2，ε3 is linear, nonlinear and high order dielectric constant respectively . When the BST system is in the ferroelectric or phase transition state, the dom ain and domain wall's extrinsic contributions to the nonlinear dielectric consta nt cannot be neglected, and the extrinsic contributions decrease with the increa se of the strength of the electric field. For the relaxor-like BZT system, even if it is in paraelectric state, the extrinsic contribution of polar microregion s to the nonlinear dielectric constant can be markedly observed. With the incre ase of the strength of the electric field and temperature, this contribution dec reases.