  We investigate MeV-GeV $\gamma$-ray outbursts in 3C 273 in the frame of a
time-dependent one-zone synchrotron self-Compton (SSC) model. In this model,
electrons are accelerated to extra-relativistic energy through the stochastic
particle acceleration and evolve with the time, nonthermal photons are produced
by both synchrotron and inverse Compton scattering of synchrotron photons.
Moreover, nonthermal photons during a quiescent are produced by the
relativistic electrons in the steady state and those during a outburst are
produced by the electrons whose injection rate is changed at some time
interval. We apply the model to two exceptionally luminous $\gamma$-ray
outbursts observed by the Fermi-LAT from 3C 273 in September, 2009 and obtain
the multi-wavelength spectra during the quiescent and during the outburst
states, respectively. Our results show that the time-dependent properties of
outbursts can be reproduced by adopting the appropriate injection rate function
of the electron population.
