  We discuss the role of the axial $U(1)_A$ symmetry in the chiral phase
transition using the $U(N_f)_R\times U(N_f)_L$ linear sigma model with two
massless quark flavors. We expect that above a certain temperature the axial
$U(1)_A$ symmetry will be effectively restored as well as $SU(N_f)_R\times
SU(N_f)_L$. Then we can construct a string-like static solution the $\eta$
string and a kink-like classical solution the domain wall during the chiral
phase transition. We give out the possible signals for detecting the domain
wall in ultrarelativistic heavy-ion collisions.
