  The phase structure of QCD with imaginary chemical potential provides
information on the phase diagram of QCD with real chemical potential. With
imaginary chemical potential $i\mu_I=i\pi T$, previous studies show that the
Roberge-Weiss (RW) transition end points are triple points at both large and
small quark masses, and second order transition points at intermediate quark
masses. The triple and second order end points are separated by two tricritical
ones. We present simulations with $ N_f=2 $ Wilson fermions to investigate the
nature of RW transition end points. The simulations are carried out at 8 values
of the hopping parameter $\kappa$ ranging from 0.020 to 0.140 on different
lattice volumes. The Binder cumulant, susceptibility and reweighted
distribution of the imaginary part of Polyakov loop are employed to determine
the nature of RW transition end points. The simulations show that the two
tricritical points are within the range $0.070-0.080$ and $0.120-0.140$,
respectively.
