  We study here the arising of Kelvin--Helmholtz Instability (KHI) in one fast
jet of 2014 April 16 observed by the Atmospheric Imaging Assembly (AIA) on
board Solar Dynamics Observatory (SDO) in different UV and EUV wavelengths. The
evolution of jet indicates the blob like structure at its boundary which could
be the observational evidence of the KHI. We model the jet as a moving
cylindrical magnetic flux tube of radius $a$ embedded in a magnetic field B_i
and surrounded by rest magnetized plasma with magnetic field B_e. We explore
the propagation of the kink MHD mode along the jet that can become unstable
against the KHI if its speed exceeds a critical value. Concerning magnetic
fields topology we consider three different configurations, notably of (i)
spatially homogeneous magnetic fields (untwisted magnetic flux tube), (ii)
internal (label `i') twisted magnetic field and external homogeneous one (label
`e') (single-twisted flux tube), and (iii) both internal and external twisted
magnetic fields (double-twisted magnetic flux tube). Plasma densities in the
two media rho_i and rho_e are assumed to be homogeneous. The density contrast
is defined in two ways: first as rho_e/rho_i and second as rho_e/(rho_i +
rho_e). Computations show that the KHI can occur at accessible flow velocities
in all the cases of untwisted and single-twisted flux tubes. It turns out,
however, that in the case of a double-twisted flux tube the KHI can merge at an
accessible jet speed only when the density contrast is calculated from the
ratio rho_e/(rho_i} + rho_e). Evaluated KHI developing times and kink mode wave
phase velocities at wavelength of 4 Mm lie in the ranges of 1--6.2 min and
202--271 km/s, respectively---all being reasonable for the modeled jet.
