  We analyze two simultaneous \nustar{} and \swift{} data of the Atoll type
neutron star (NS) X-ray binary 4U~1728--34 observed on 1 and 3 October, 2013.
Based on power density spectra, hardness ratio and colour-colour diagram, we
infer that the first and second observations belong to the island state and
lower banana state, respectively. During island state, a low-frequency
quasi-periodic oscillation (LFQPO) at $\sim4.53$ Hz is observed along with four
type-I X-ray bursts ($\sim 10-15$ s duration each). The X-ray luminosity of the
source during the island and lower banana states are found to be $L_{X}= 1.1$
and $1.6\times10^{37}$ erg s$^{-1}$, respectively which correspond to $\sim
6\%$ and $\sim 9\%$ of the Eddington luminosity. The combined burst spectrum is
well represented by a blackbody with a characteristic temperature of
$2.22\pm0.05\kev$ and the blackbody radius of $9.87\pm1.86$ km which is
consistent with the typical radius of the neutron star. The persistent energy
spectra from both observations in the energy band $1-79\kev$ are well described
with thermal emission from the NS surface ($kT_{bb}\simeq1-2.5\kev$),
Comptonized emission of thermal seed photons from the hot boundary layer/corona
and the strong reflection component from the accretion disc. We detect a broad
Iron line in the $5-8 \kev$ band and reflection hump in the $15-30 \kev$ band.
These features are well modelled by the {\tt relxill} reflection model. From
joint spectral fitting, we constrain the inclination angle of the binary system
and inner disc radius to be $22\degree-40\degree$ and $(2.8-4.3)\times
R_{ISCO}$, respectively. From inner disc radius, we estimate the magnetic field
to be $(3.3-6.5)\times10^{8}$ Gauss.
