This paper presents a broadband wide-angle polarizer based on a field transformation (FT) method. By adjusting the thickness of such a polarizer, we achieve polarization conversions between TE and TM, LCP and RCP, linear and circular polarizations respectively for both transmission and reflection cases. Such a polarizer can be realized through sub-wavelength dielectric gratings based on the effect media theory, their performance have been verified through full-wave numerical simulations and experiments in microwave regime.