  This study investigates the influence of the fourth generation quarks on the
total branching ratio and the single lepton polarizations in \Lambda_b -->
\Lambda \ell^+ \ell^- decay. Taking $|V_{t's}V_{t'b}| {0.01-0.03} with phase
just below $90^\circ$, which is consistent with the b\to s\ell^+\ell^- rate and
the B_s mixing parameter \Delta m_{B_s}, we obtain that the total branching
ratio and the single lepton(\mu, \tau) polarizations are quite sensitive to the
existence of fourth generation. It can serve as a good tool to search for new
physics effects, precisely, to search for the fourth generation quarks(t', b').
