  We perform a simulation with Galacticus, a semi-analytical galaxy formation
model, to predict the number counts of H$\alpha$ and [OIII] emitting galaxies.
With a state-of-the-art N-body simulation, UNIT, we first calibrate Galacticus
with the current observation of H$\alpha$ luminosity function. The resulting
model coupled with a dust attenuation model, can reproduce the current
observations, including the H$\alpha$ luminosity function from HiZELS and
number density from WISP. We extrapolate the model prediction to higher
redshift and the result is found to be consistent with previous investigations.
We then use the same galaxy formation model to predict the number counts for
[OIII] emitting galaxies. The result provides further validation of our galaxy
formation model and dust model. We present number counts of H$\alpha$ and
[OIII] emission line galaxies for three different line flux limits:
$5\times10^{-17}$erg/s/cm$^{2}$, $1\times10^{-16}$erg/s/cm$^{2}$ (6.5$\sigma$
nominal depth for WFIRST GRS), and $2\times10^{-16}$erg/s/cm$^{2}$ (3.5$\sigma$
depth of Euclid GRS). At redshift $2<z<3$, our model predicts that WFIRST can
observe hundreds of [OIII] emission line galaxies per square degree with a line
flux limit of $1\times10^{-16}$erg/s/cm$^{2}$. This will provide accurate
measurement of large scale structure to probe dark energy over a huge cosmic
volume to an unprecedented high redshift. Finally, we compare the flux ratio of
H$\alpha$/[OIII] within the redshift range of $0<z<3$. Our results show the
known trend of increasing H$\alpha$/[O III] flux ratio with H$\alpha$ flux at
low redshift, which becomes a weaker trend at higher redshifts.
