A sharp (several orders of magnitude) increase of chain mobility was observed at a certain temperature in molecular dynamics simulation of the polyethylene crystal. Chain mobility as a function of temperature in a polyethylene perfect crystalline structure was studied by applying molecular dynamics simulation with COMPASS and flexible Williams force fields. Three crystallographic crystalline phases of polyethylene were observed in the simulations in both force fields: orthorhombic, monoclinic, and conformationally disordered (condis) phases. There is no three-dimensional crystallographic order in the condis phase-only two-dimensional translational symmetry, which was observed in the plane perpendicular to the chain direction. The simulations show that the chain mobility undergoes a dramatic increase by several orders of magnitude at the transition from monoclinic to condis phase of the crystal. In the condis phase, mobility is almost independent of temperature, which indicates that the diffusion activation energy is less than kT. The calculated mobility in the condis phase is almost the same in both force fields. The calculated kinetic energy of the system per C-C bond at the temperature of the monoclinic -> condis transition is approximately equal to the barrier of torsion (trans-eclipsed-trans) around the C-C bond.