  This study investigates the influence of the fourth generation quarks on the
double lepton polarizations in \Lambda_b --> \Lambda \ell^+ \ell^- decay by
taking |V^\ast_{t's}V_{t'b}|= 0.005,0.01,0.02,0.03 with phase
\{60^\circ,90^\circ,120^\circ\}. We will try to obtain a constrain on the mass
of the 4th generation top like quark t', which is consistent with the $b-->
s\ell^+\ell^- rate . With the above mentioned parameters, we will try to show
that the double lepton(muon, 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')
via its indirect manifestations in loop diagrams.
