  Because of the disappearance of its accretion disk since the time period
around 2012 November--December, XSS J12270-4859 has recently been identified
as, in addition to PSR J1023+0038, another transitional millisecond pulsar
binary. We have carried out detailed analysis of the Fermi Large Area Telescope
data for the binary. While both spectra before and after the disk-disappearance
transition are well described by an exponentially cut-off power law, typical
for pulsars' emission in the Fermi's 0.2-300 GeV, a factor of 2 flux decrease
related to the transition is detected. A weak orbital modulation is seen, but
only detectable in the after-transition data, same to that found at X-rays. In
the long-term light curve of the source before the transition, a factor of 3
flux variations are seen. Comparing to the properties of J1023+0038, we disucss
the implications from these results. We suggest that since the modulation is
aligned with that at X-rays in orbital phase, it possibly arises due to the
occultation of the gamma-ray emitting region by the companion. The origin of
the variations in the long-term light curve is not clear, because the source
field also contains unidentified radio or X-ray sources and their contamination
can not be excluded. Multi-wavelength observations of the source field will
help identify the origin of the variations by detecting any related flux
changes from the in-field sources.
