  We report the extended GeV $\gamma$-ray emission that spatially associated
with the high Galactic latitude supernova remnant (SNR) candidate - Calvera's
SNR with the Pass 8 data recorded by the {\em Fermi} Large Area Telescope. The
$\gamma$-ray spectrum of Calvera's SNR between 100 MeV and 1 TeV shows an
evident ($\sim$ 3.4$\sigma$) spectral curvature at several tens of GeV. The
multi-wavelength data can be fitted with either a leptonic model or a hadronic
one. However, the leptonic model exhibits the inconsistent between the flat
radio spectrum and the hard GeV $\gamma$-ray spectrum of Calvera's SNR. For the
hadronic model, the spectral index of protons should be harder than 1.6. And
the total energy of protons is fitted to be more than one order of magnitude
higher than the explosion energy of a typical supernova, which also challenges
the hadronic model. The evident spectral curvature and the absence of
non-thermal X-ray emission from Calvera's SNR makes it to be an interesting
source bridging young-aged SNRs with bright non-thermal X-ray emission and
old-aged SNRs interacting with molecular clouds.
