  In this paper, we used the Monte Carlo code CRPropa version 2.0 to simulate
the propagation of UHE cosmic rays. In the simulation, uniform cosmic ray
sources with spectral indices of {\alpha} = 1.4, {\alpha} = 2 and Emax = 1021
eV for pure iron composition at the source position are assumed. We obtained
the logarithmic average mass of cosmic rays as a function of energy and
compared it with the experimental values in the ankle, and we demonstrated how
the mass composition changes before and after the ankle.
