  HH 399 is one of the first Herbig Haro flows recognized to be irradiated by
the UV radiation of the massive O7.5 star in the Trifid nebula. We present the
proper motion of the first irradiated jet based on two epochs of HST
observations of HH 399 separated nearly by five years using H$\alpha$ and [SII]
line filters. High proper motion with continuous velocities between 200$\pm$55
and 528$\pm24$ \kms are detected in both lines along the 18$''$ extent of the
jet axis. The irradiated fully-ionized jet consists of numerous knots along the
jet but also shows the evidence for a number of isolated blob-like structures
running immediately outside the jet with lower transverse velocities. The
transverse velocities combined with radial velocity measurements indicate that
the jet axis lies away from the plane of the sky by only few degrees. We argue
that the jet is fully ionized based on [SII]/H$\alpha$ line ratio as well as
radio continuum emission detected from the full extent of the jet at 3.6cm
wavelength. The stellar mass-loss rate producing HH 399 is estimated to be
$\approx 2\times10^{-6} \msol $yr$^{-1}$.
