In this paper,a linear-to-circular polarization converter using a three-layer frequency selective surface based on Ⅰ-shaped circular structure resonant is presented and investigated.Numerical simulations exhibit that when the normal ypolarized waves impinge on this device propagating towards +z direction,the two orthogonal components of the transmitted waves have a 90° phase difference as well as the nearly equal amplitudes at the resonant frequency of 7.04 GHz,which means that the left-hand circular polarization is realized in transmission.For validating the proposed design,a prototype which consists of 25 × 25 elements has been designed,manufactured and measured.The measured results are in good agreement with the simulated ones,showing that the polarization conversion transmission is over-3 dB in the frequency range of 5.22-8.08 GHz and the axial ratio is below 3 dB from 5.86 GHz to 7.34 GHz.