Abstract: A preternatural and extremely thin metasurface with weak asymmetric unit structure is presented here to demonstrate extraordinary strong chirality. The unit cell of metasurface is composed of a double layer of elliptical metal patches with a certain twisted angle and a medium sandwiched between them. When the twisted angle equals to 80°, optical activity can be realized in this metasurface. At the resonant frequency 11.89 GHz, the incident linearly polarized wave is converted into its cross-polarization wave with the transmittance rate higher than 94%. The light weight and miniaturization of this metasurface provide a reliable approach for polarization manipulation. If extended to light waveband, the metasurface may have potentials in biological applications such as detection of weak chiral molecules, etc.