  The finite temperature QCD phase transition is studied on the lattice with
degenerate two flavors of Wilson quarks. Motivated by reported strange
behaviors with the standard action on lattices with the temporal extention
$N_t=4$ and 6, a renormalization group improved gauge action is applied. On an
$N_t=4$ lattice, the strange behaviors observed with the standard action are
removed with our improved action. The finite temperature transition is
continuous in the chiral limit and it becomes quite smooth in all observables
we studied when we increase the quark mass by increasing $\beta$ along the
crossover line.
