  Temperature inversion layers are predicted to be present in ultra-hot giant
planet atmospheres. Although such inversion layers have recently been observed
in several ultra-hot Jupiters, the chemical species responsible for creating
the inversion remain unidentified. Here, we present observations of the thermal
emission spectrum of an ultra-hot Jupiter, WASP-189b, at high spectral
resolution using the HARPS-N spectrograph. Using the cross-correlation
technique, we detect a strong Fe I signal. The detected Fe I spectral lines are
found in emission, which is direct evidence of a temperature inversion in the
planetary atmosphere. We further performed a retrieval on the observed spectrum
using a forward model with an MCMC approach. When assuming a solar metallicity,
the best-fit result returns a temperature of $4320_{-100}^{+120}$ K at the top
of the inversion, which is significantly hotter than the planetary equilibrium
temperature (2641 K). The temperature at the bottom of the inversion is
determined as $2200_{-800}^{+1000}$ K. Such a strong temperature inversion is
probably created by the absorption of atomic species like Fe I.
