  Our knowledge of Giga-electron volt (GeV) radio galaxies has been
revolutionized by the Fermi-LAT Telescope, which provides an excellent
opportunity to study the physical properties of GeV radio galaxies. According
to the radio power and morphology, radio galaxies can be separated into
Fanaroff-Riley Type I radio galaxies (FR-Is) and Type II radio galaxies
(FR-IIs). In this paper, we consider the unification of FR-Is and BL Lacertae
objects (BL Lacs), and assume FR-Is to be a standard candle to discuss the
beaming effect for Fermi-LAT-detected FR-Is. Our main conclusions are as
follows: (1) The estimated Doppler factors for 30 Fermi-LAT-detected FR-Is are
in a range of $\delta_{\rm{I}}=0.88-7.49$. The average Doppler factor
($<\delta_{\rm{I}}>=2.56\pm0.30$) of the 30 FR-Is is smaller than that
($<\delta_{\rm{BL}}>=10.28\pm2.03$) of the 126 Fermi-LAT-detected BL Lacs,
supporting the unification model that FR-Is are regarded as the misaligned BL
Lacs with smaller Doppler factors; (2) We propose that different regions of
FR-Is in the plot of the $\gamma$-ray luminosity against the photon spectral
index $(\log L_{\gamma}-\alpha_{\rm{ph}})$ may indicate the different beaming
effects; (3) The average Doppler factor of the 6 TeV FR-Is is similar to that
of the 24 non-TeV FR-Is, which implies that the difference between the TeV and
GeV emissions is not driven by the beaming effect in the Fermi-LAT-detected
FR-I samples.
