The impedance and dielectric properties of Ag/Al2O3 composites are investigated experimentally in the frequency range from 100 MHz to 1 GHz. Besides, equivalent circuit analysis and numerical simulations were carried out. For the composites with sufficiently high silver loading, current paths were formed and negative permittivity appeared. The negative permittivity can be well described by lossy Drude model. Moreover, the negative permittivity sample manifests inductive characteristic and shunt inductors are added to its equivalent circuit. Numerical simulations show that the interconnection of silver particles results in negative permittivity, hence the serious attenuation of electromagnetic waves.