The dynamical reversal of rectangular CoFe nanodot with initial C-state subject to square magnetic field pulse has been studied by using the micromagnetic simulations. It was found that the reversals proceeded through different modes as the pulse amplitude is increased. For the weak magnetic field pulse， the magnetization reversal is realized through the motion of the end domain walls toward the central part of magnet and， subsequently， the formation and motion of a single vortex. As the strength of magnetic field pulse become stronger， the reversal is accomplished through the motion of the end domain walls plus the formation and motion of two vortex structure. For much stronger field pulse the reversal process is the motion of the end domain walls and subsequent formation and annihilation of several vortices. The dependence of reversal time on the field pulse amplitude displays oscillating characteristic due to the transitions of reversal modes with the field strength.