  The absolute branching fraction of $D^+ \to \mu^+ \nu$ has been directly
measured by an analysis of a data sample of about 33 ${\rm pb^{-1}}$ collected
around $\sqrt{s}=3.773$ GeV with the BES-II at the BEPC. At these energies,
$D^-$ meson is produced in pair as $e^+e^-\to D^{+} D^{-}$. A total of $5321
\pm 149 \pm 160$ $D^-$ mesons are reconstructed from this data set. In the
recoil side of the tagged $D^-$ mesons, $2.67\pm1.74$ purely leptonic decay
events of $D^+ \to \mu^+ \nu$ are observed. This yields a branching fraction of
$BF(D^+ \to \mu^+ \nu_{\mu}) = (0.122^{+0.111}_{-0.053}\pm 0.010)%$, and a
corresponding pseudoscalar decay constant $f_{D^+}=(371^{+129}_{-119}\pm 25)$
MeV.
