  The weakly interacting massive particle (WIMP) often serves as a candidate
for the cold dark matter, however when produced non-thermally it could behave
like warm dark matter. In this paper we study the properties of the
$\gamma$-ray emission from annihilation of WIMP dark matter in the halo of our
own Milky-Way Galaxy with high resolution $N$-body simulations of a Milky-Way
like dark matter halo, assuming different nature of WIMPs. Due to the large
free-streaming length in the scenario of warm WIMPs, the substructure contend
of the dark matter halo is significantly different from that of the cold WIMP
counterpart, resulting in distinct predictions of the $\gamma$-ray signals from
the dark matter annihilation. We illustrate these by comparing the predicted
$\gamma$-ray signals from the warm WIMP annihilation to that of cold WIMPs.
Pronounced differences from the subhalo skymap and statistical properties
between two WIMP models are demonstrated. Due to the potentially enhanced cross
section of the non-thermal production mechanism in warm WIMP scenario, the
Galactic center might be prior for the indirect detection of warm WIMPs to
dwarf galaxies, which might be different from the cold dark matter scenario. As
a specific example we consider the non-thermally produced neutralino of
supersymmetric model and discuss the detectability of warm WIMPs with Fermi
$\gamma$-ray telescope.
