  It was found that the equatorial velocity of A-type stars undergoes an
acceleration in the first third of the main sequence (MS) stage, but the
velocity decreases as if the stars were not undergoing any redistribution of
angular momentum in the external layers in the last stage of the MS phase. Our
calculations show that the acceleration and the decrease of the equatorial
velocity can be reproduced by the evolution of the differential rotation
zero-age MS model with the angular momentum transport caused by hydrodynamic
instabilities during the MS stage. The acceleration results from the fact that
the angular momentum stored in the interiors of the stars is transported
outwards. In the last stage, the core and the radiative envelope are
uncoupling, and the rotation of the envelope is a quasi-solid rotation; the
uncoupling and the expansion of the envelope lead to that the decrease of the
equatorial velocity approximately follows the slope for the change in the
equatorial velocity of the model without any redistribution of angular
momentum. When the fractional age 0.3 $\lesssim\mathrm{t/t_{MS}}\lesssim$ 0.5,
the equatorial velocity remains almost constant for the stars whose central
density increases with age in the early stage of the MS phase, while the
velocity decreases with age for the stars whose central density decreases with
age in the early stage of the MS phase.
