  We investigate some basic properties of Damped Lyman alpha systems based on
the Semi-Analytical model of disk galaxy formation theory. We derive the DLA
metallicity, column density, number density, gas content and cosmic star
formation rate by assuming that disks form at the center of dark halos, and the
modelled DLAs are selected by Monte Carlo simulation according to the
distributions of halo properties. We find that DLA hosts are dominated by small
galaxies and biased to extended galaxies. In terms of model results, DLAs could
naturally arise in a $\Lambda$CDM universe from radiatively cooled gas in dark
matter halos. However, model predicts a reverse correlation between metallicity
and the column density when compared with observations, regardless of the
proposed observational bias. We argue that this could be resulted from the
model limitations, or the inadequacy of Schmidt-type star formation mode at
high redshift, or/and the diversities of DLA populations.
