  We investigate the properties of quarks and gluons above the chiral phase
transition temperature $T_c,$ using the RG improved gauge action and the Wilson
quark action with two degenerate quarks mainly on a $32^3\times 16$ lattice. In
the one-loop perturbation theory, the thermal ensemble is dominated by the
gauge configurations with effectively $Z(3)$ center twisted boundary
conditions, making the thermal expectation value of the spatial Polyakov loop
take a non-trivial $Z(3)$ center. This is in agreement with our lattice
simulation of high temperature QCD. We further observe that the temporal
propagator of massless quarks at extremely high temperature $\beta=100.0 \, (T
\simeq10^{58} T_c)$ remarkably agrees with the temporal propagator of free
quarks with the $Z(3)$ twisted boundary condition for $t/L_t \geq 0.2$, but
differs from that with the $Z(3)$ trivial boundary condition. As we increase
the mass of quarks $m_q$, we find that the thermal ensemble continues to be
dominated by the $Z(3)$ twisted gauge field configurations as long as $m_q \le
3.0 \, T$ and above that the $Z(3)$ trivial configurations come in. The
transition is essentially identical to what we found in the departure from the
conformal region in the zero-temperature many-flavor conformal QCD on a finite
lattice by increasing the mass of quarks. We argue that the behavior is
consistent with the renormalization group analysis at finite temperature.
