Based on effective permittivity theory of heterogeneous material, the effective complex permittivity of two-dimensional subwavelength Ag particle/hole has been simulated. The optical resonance properties of Ag particle/hole are verified. For the Ag particle-dielectric composite structure, the effective permittivity has a negative real part when the optical frequency is larger than the plasmon frequency of Ag. The imaginary part of the effective permittivity shows a resonance near the plasmon frequency of Ag, and the resonance peak displays a red shift with the increase of radius of Ag particle. By calculating the electric field distribution, it is found that optical resonance results in very strong local field enhancements around the Ag particle at the resonant frequency. Similar phenomena are observed for the structure of Ag block with a single hole filled with different materials, and resonant peak shows red shift with the increase of permittivity in the hole, while the resonant peak is blue-shifted with the increase of radius of the hole filled with the same material. In addition, the optical resonance can be controlled by adjusting the size and geometry of the particle/hole. Therefore, the mechanisms for the interaction of light with particle/hole are surface plasmon mode and resonant mode, and the effective-medium theory is a powerful tool for exploring the intriguing enhanced optical transmission phenomena.