We present the results of a numerical study comparing photometric and physical properties of simulated z= 6–9 galaxies to the observations taken by the Wide Field Camera 3 instrument aboard the Hubble Space Telescope. Using cosmological hydrodynamical simulations we find good agreement with observations in colour–colour space at all studied redshifts. We also find good agreement between observations and our Schechter luminosity function fit in the observable range, Muv≤−18, provided that a moderate dust extinction effect exists for massive galaxies. However beyond what currently can be observed, simulations predict a very large number of low-mass galaxies and evolving steep faint-end slopes from αL=−2.15 at z= 6 to αL=−2.64 at z= 9, with a dependence of |αL| ∝ (1 +z)0.59. During the same epoch, the normalization ϕ* increases and the characteristic magnitude becomes moderately brighter with decreasing redshift. We find similar trends for galaxy stellar mass function with evolving low-mass end slope from αM=−2.26 at z= 6 to αM=−2.87 at z= 9, with a dependence of |αM| ∝ (1 +z)0.65. Together with our recent result on the high escape fraction of ionizing photons for low-mass galaxies, our results suggest that the low-mass galaxies are important contributor of ionizing photons for the reionization of the Universe at z≥ 6.