  The recent measurement on the decay constant of $D_s$ shows a discrepancy
between theory and experiment. We study the leptonic and semileptonic decays of
$D$ and $D_s$ simultaneously within the standard model by employing a
lightfront quark model. There is space by tuning phenomenological parameters
which can explain the "$f_{D_s}$ puzzle" and do not contradict other
experiments on the semileptonic decays. We also investigate the leptonic decays
of D and $D_{s}$ with a new physics scenario, unparticle physics. The
unparticle effects induce a constructive interference with the standard model
contribution. The nontrivial phase in unparticle physics could produce direct
CP violation which may distinguish it from other new physics scenarios.
