CP asymmetries in $B \to \phi K_S$ decay  is studied in a special
context of supersymmetry theories, in which the charginos play an
important role. We find that except gluino, chargino can also give
significant contributions to $B \to \phi K_S$ decay. After considering
the constraints from $B \to J/\psi K_S$ decay, we study three special
scenarios:  
(a). Large mixing on left-handed charm and top squarks(LL mixing); 
(b). Large mixing on right-handed charm and top squarks (RR mixing); 
(c). Large mixing both on left-handed charm-top squark and right-handed
     charm-top squark(LL+RR mixing). 
We show quantitatively that because of large squark mixing within second
and third generations, an $\Oi$(1) effect on CP violation in $B \to \phi
K_S$ is possible.