  Recently the LHAASO Collaboration published the detection of 12
ultra-high-energy gamma-ray sources above 100 TeV, with the highest energy
photon reaching 1.4 PeV. The first detection of PeV gamma rays from
astrophysical sources may provide a very sensitive probe of the effect of the
Lorentz invariance violation (LIV), which results in decay of high-energy gamma
rays in the superluminal scenario and hence a sharp cutoff of the energy
spectrum. Two highest energy sources are studied in this work. No signature of
the existence of LIV is found in their energy spectra, and the lower limits on
the LIV energy scale are derived. Our results show that the first-order LIV
energy scale should be higher than about 10^5 times the Planck scale M_{pl} and
that the second-order LIV scale is >10^{-3}M_{pl}. Both limits improve by at
least one order of magnitude the previous results.
