We present the full nonlinear calculation of the formation of a Q ball thro
ugh the Affleck-Dine (AD) mechanism by numerical simulations. It is shown t
hat large Q balls are actually produced by the fragmentation of the condens
ate of a scalar field whose potential is very flat. We find that the typica
l size of a Q ball is determined by the most developed mode of linearized f
luctuations, and almost all the initial charges which the AD condensate car
ries are absorbed into the formed Q balls, whose sizes and the charges depe
nd only on the initial charge densities.