  We present the near-infrared (Ks-band) luminosity function of galaxies in two
z~1 cluster candidates, 3C336 and Q1335+28. A third cluster, 3C289, was
observed but found to be contaminated by a foreground system. Our wide field
imaging data reach to Ks=20.5 (5sigma), corresponding to ~M*+2.7 with respect
to the passive evolution. The near-infrared luminosity traces the stellar mass
of a galaxy due to its small sensitivity to the recent star formation history.
Thus the luminosity function can be transformed to the stellar mass function of
galaxies using the $J-K$ colours with only a small correction (factor<2) for
the effects of on-going star formation. The derived stellar mass function spans
a wide range in mass from ~3 x 10^{11}Msun down to ~6 x 10^{9}Msun (set by the
magnitude limit). The form of the mass function is very similar to lower
redshift counterparts such as that from 2MASS/LCRS clusters (Balogh et al.
2001) and the z=0.31 clusters (Barger et al. 1998). This indicates little
evolution of galaxy masses from z=1 to the present-day. Combined with colour
data that suggest star formation is completed early (z>>1) in the cluster core,
it seems that the galaxy formation processes (both star formation and mass
assembly) are strongly accerelated in dense environments and has been largely
completed by z=1. We investigate whether the epoch of mass assembly of massive
cluster galaxies is earlier than that predicted by the hierarchical galaxy
formation models. These models predict the increase of characteristic mass by
more than factor ~3 between z=1 and the present day. This seems incompatible
with our data.
