  Despite its small size, the asteroid 4 Vesta has been completely
differentiated to core and mantle. Its composition is similar to
howardite--eucrite--diogenite (HED) meteorites of which the detailed petrology
is known. Therefore, Vesta is a good target for understanding the
differentiation of terrestrial planets. A new differentiation model for crust
formation has been developed by taking magma ocean fluid dynamics, chemical
equilibrium, the presence of $^{26}$Al, and cooling into consideration with a
special focus on crystal separation. The role of crystal size, thickness of the
conductive lid, and fO$_{2}$ are evaluated as parameters. The results show that
large crystals of at least 1 cm settled and formed a kilometer-thick cumulate
layer of orthopyroxene with Mg$\#$ of 0.70-0.90 in ~20 thousand years, which
almost agrees with the Mg$\#$ of diogenites. Smaller grain sizes formed thinner
layers.
