  We study the rare decays of $B_c^+\to D_{q}^{(*)+}l\bar{l}$ ($q=d,s$ and
$l=\nu_l,e,\mu,\tau)$ in the standard model. The form factors are evaluated in
the light front and constituent quark models, respectively. We find that the
decay branching ratios calculated in the two models for $B_c^+\to D^+_q
l\bar{l}$ agree well with each other, whereas those for $B_c^+\to
D_q^{*+}l\bar{l}$ are different.
