We investigate the effects of the UV background radiation on galaxy formation by using the semi-analytic model including the photoionization process .
The semi-analytic model is based on Cole et al .
and we use almost the same parameters as their ‘ fiducial ’ model .
We find that the UV background mainly affects the formation of dwarf galaxies when J _ { -21 } \ga 1 ( J \equiv J _ { -21 } \times 10 ^ { -21 } ergs cm ^ { -2 } s ^ { -1 } sr ^ { -1 } Hz ^ { -1 } ,J is the intensity of the UV background ) .
Because of the suppression of star formation , the number density of small objects corresponding to dwarf galaxies decreases compared with the case of no UV radiation when the UV background exists until the present epoch .
On the contrary , the UV radiation hardly affects massive galaxies .
This is because the massive galaxies are formed by mergers of small galaxies , which are formed at high redshift when the effect of the UV background is negligible .
This strongly suggests that it is important to consider the merging histories of the galaxies .
On the other hand , when the UV background vanishes at a low redshift ( z \sim 2 ) , the number density of small objects is hardly changed but the colour becomes bluer , compared with the case of no UV radiation , because stars are newly formed after the UV background vanishes .

Moreover , we show the redshift evolutions of the luminosity functions and the colour distributions of galaxies .
Because the effect of the UV background is strong at low redshift , we can discriminate between the types of evolution of the UV background by observing the evolution of the luminosity function and the colour distributions , if the UV intensity is sufficiently strong ( J _ { -21 } \ga 1 ) .