In this paper, active frequency selective surfaces (FSS) with squared aperture with loading are described. Active FSS elements using switched PIN diodes are discussed. The unit cell consists of a square aperture element with a metal island loading and one PIN diode placed at the upper gap considering the vertical polarization. The electromagnetic properties of the active FSS structure are changed by applying a dc bias to the substrate, and estimated by the equivalent circuit model of FSS structure and PIN diode. This active FSS design enables transmission to be switched on or off at 2.3 GHz, providing high transmission when the diodes are in off state and high isolation when the diodes are on. Equivalent circuit in the structure is investigated by analyzing transmission and reflection spectra. Measurements on active FSS are compared with numerical calculation based on the scattering matrix formalism. The origin of experimentally observed frequency responses will be scrutinized.