  In the framework of the top triangle moose $(TTM)$ model, we analyze the rare
decays $\Lambda_{b}\rightarrow \Lambda l^{+}l^{-} (l=e,\mu,\tau)$ by using the
form factors calculated in full $QCD$. We calculate the contributions of the
new particles predicted by this model to observables, such as the branching
ratios, forward-backward asymmetry ($A_{FB}$) and polarizations related these
decay processes. We find that, in wide range of the parameter space, the values
of the branching ratios are enhanced by one order of magnitude comparing to the
$SM$ predictions. This model can also produce significant corrections to
$A_{FB}$, normal polarization $P_N$ and transversal polarization $P_T$.
