  In a covariant quark-diquark model, we investigate the rare decay of
$\Lambda_b \rightarrow n l^+ l^-$ and $\Lambda_b \rightarrow n \gamma$ in the
Bethe-Salpeter equation approach. In this model the baryons are treated as
bound states of a constituent quark and a diquark interacting via a gluon
exchange and the linear confinement. We find that the ratio of form factors $R$
is varies from $-0.90$ to $-0.25$ and the branching ratios $Br(\Lambda_b
\rightarrow n l^+ l^-)\times 10^{8}$ are $6.79(l=e),~4.08 ~ (l=\mu),~2.9
~(l=\tau) $ and the branching ratio $Br(\Lambda_b \rightarrow \gamma)\times
10^{7} )=3.69$ in central values of parameters.
