  We study a scattering problem of gravitational waves (GWs) by an axion domain
wall in Chern-Simons (CS) gravity. We find that circular polarization of GWs is
produced after passing through the domain wall. It turns out that the circular
polarization is sizable if the frequency of the GW is comparable to a critical
value determined by the characteristic CS length scale and the energy scale of
the axion domain wall. Thus, observations of the circular polarization could
give a stringent constraint on the characteristic CS length scale or could be a
new avenue to search for axion.
