  We investigate the evolution of the galaxy stellar mass function at
high-redshift ($z\ge 5$) using a pair of large cosmological hydrodynamical
simulations: {\em MassiveBlack} and {\em MassiveBlack-II}. By combining these
simulations we can study the properties of galaxies with stellar masses greater
than $10^{8}\,{\rm M_{\odot}}\,h^{-1}$ and (co-moving) number densities of
$\log_{10}(\phi\, [{\rm Mpc^{-3}\,dex^{-1}}\,h^{3}])>-8$. Observational
determinations of the galaxy stellar mass function at very-high redshift
typically assume a relation between the observed UV luminosity and stellar
mass-to-light ratio which is applied to high-redshift samples in order to
estimate stellar masses. This relation can also be measured from the
simulations. We do this, finding two significant differences with the usual
observational assumption: it evolves strongly with redshift and has a different
shape. Using this relation to make a consistent comparison between galaxy
stellar mass functions we find that at $z=6$ and above the simulation
predictions are in good agreement with observed data over the whole mass range.
Without using the correct UV luminosity and stellar mass-to-light ratio, the
discrepancy would be up to two orders of magnitude for large galaxies
$>10^{10}\,{\rm M_{\odot}}\,h^{-1}$. At $z=5$, however the stellar mass
function for low mass $<10^{9}\,{\rm M_{\odot}}\,h^{-1}$ galaxies is
overpredicted by factors of a few, consistent with the behaviour of the UV
luminosity function, and perhaps a sign that feedback in the simulation is not
efficient enough for these galaxies.
