We demonstrate experimentally and numerically that a bilayer triangular-shaped chiral metamaterial (TCMM) gives a nearly perfect cross-polarization with 90% polarization conversion efficiency reported at X-band regime. The giant optical activity in the proposed TCMM arises due to the strong cross-coupling between the electric and magnetic dipole as a result of generated strong rotatory-power for the normally incident electromagnetic-wave. It is showed that TCMM can convert the linearly polarized wave into right/left circularly polarized wave. A chiral resonance at 10.32GHz rotates y-polarized electric field into x-polarized field with almost 90% transmission magnitude. Right and left circularly wave possesses different refractive-indices due to presence of chirality-factor (κ) in optically active TCMM metamaterial. The mechanism of cross-coupling is understood based on surface current distribution on the metallic patterns. The proposed polarization converter may be regarded as potential polarizing elements in optical polarization spectroscopy and microwave photonics applications with different length-scale. © 2016 European Association of Antennas and Propagation.