  In the present paper, we systematically studied the possible Orbital
selective Mott transition (OSMT) in the $t_{2g}$ system, which has three
orbital degeneracy. The slave Boson mean field theory is generalized to the
three band systems with full Hund's rule coupling including spin flip and pair
hopping terms. A new type of Mott transition which is driven by the crystal
field splitting or the lattice distortion is found is this model. We argue that
this new type of Mott transition may relate to the puzzling phase in
$Sr_{x}Ca_{2-x}RuO_4$.
