We investigate the nature of the chiral phase transition in the massless two-flavor QCD using the renormalization group improved gauge action and the Wilson quark action on $32^3times 16$, $24^3times 12$, and $16^3times 8$ lattices. We calculate the spacial and temporal propagators of the iso-triplet mesons in the pseudo-scalar ($PS$), scalar ($S$), vector ($V$) and axial-vector ($AV$) channels on the lattice of three sizes. We first verify that the RG scaling is excellently satisfied for all cases. This is consistent with the claim that the chiral phase transition is second order. Then we compare the spacial and temporal effective masses between the axial partners, i.e. $PS$ vs $S$ and $V$ vs $AV$, on each of the three size lattices. We find the effective masses of all of six cases for the axial partners agree remarkably. This is consistent with the claim that at least $Z_4$ subgroup of the $U_A(1)$ symmetry in addition to the $SU_A(2)$ symmetry is recovered at the chiral phase transition point.
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