  We study the effects of a vector-like SU(2) doublet quarks as the fourth
generation on $R_b$ and $R_c$. By considering the constraint from the
experimental data of forward-backward asymmetry, $A_{FB}^b$ and $A_{FB}^c$,as
well as FCNC among light quarks, we show that there is an allowed region of
parameters for $R_b$ but not for $R_c$.
