  We introduced a semi-analytic model to study the detail of radial related
processes on galaxy disks. Based on this recipe, we can calculate neutral and
molecular gas in interstellar medium, and include the molecular phase into star
formation recipes. From our model, we reproduced the radial surface density
profiles and mass functions shown in the observational results. We can also
give some predictions and explanations to the scale relations of neutral and
molecular gas in disk galaxies.
