The temperature and frequency dependent dielectric properties of polycrystalline Cd-doped Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ (y=0,0.25,0.5,0.75) bulk superconductor samples are investigated. The zero resistivity critical temperature {Tc(R=0)} has decreased and normal state resistivity has increased with the increase of Cd-doping in Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ samples. The dielectric properties such as dielectric constants (e′,e″), dielectric loss tangent (tanδ) and ac-conductivity (σac) are investigated by measuring the capacitance (C) and conductance (G) in the frequency range of 10 KHz to 10 MHz at different temperature from 80 K to 300 K. The negative capacitance (NC) is observed in all Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ samples. The large values of NC observed at lower frequencies and temperatures may be due to reduced thermal vibrations and enhanced polarizability of the material. The effect of Cd-doping on bulk properties, dc-resistivity (ρ) and ac-electrical conductivity (σac) of these superconductor samples are investigated. The polarization in Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ samples is most likely arising from the displacement of charges in CuO2/CdO2 planes relative to the static charges at Ba2+, Tl3+, and Cu2+ sites in Cu0.5Tl0.5Ba2O4−δ charge reservoir layers by external applied field.