  The $Fermi$-LAT has revealed that rotation powered millisecond pulsars (MSPs)
are a major contributor to the Galactic $\gamma$-ray source population. We
discuss the $\gamma$-ray emission process within the context of the outer gap
accelerator model, and use a Monte-Calro method to simulate the Galactic
population of the $\gamma$-ray emitting MSPs. We find that the outer gap
accelerator controlled by the magnetic pair-creation process is preferable in
explaining the possible correlation between the $\gamma$-ray luminosity and the
spin down power. Our Monte-Calro simulation implies that most of the
$\gamma$-ray emitting MSPs are radio quiet in the present sensitivity of the
radio survey, indicating that most of the $\gamma$-ray MSPs have been
unidentified. We argue that the Galactic $Fermi$ unidentified sources located
at high latitudes should be dominated by MSPs, whereas the sources in the
galactic plane are dominated by radio-quiet canonical pulsars.
