A gallium (Ga) focused-ion-beam (FIB) has been popularly used to prepare cross-sectional samples for transmission electron microscopes (TEMs) and scanning electron or ion microscopes. However, characteristics of the FIB-prepared cross sections such as ion concentration and radiation damage have been little studied either in theory or in experiment. In the present study, cross sections prepared by 30 keV Ga FIB are modeled using a combination of analytical and Monte Carlo methods to calculate the implanted Ga concentration. It is found that the Si/W layered sample is cross sectioned at grazing angles beta approximate to 2.5 degrees and 6 degrees for these layers, respectively. The implanted Ga ions for the cross-sectioned Si and W layers are concentrated very near their surfaces of < 10 nm to yield the Ga concentrations C-Ga Of about 4 and 9 at % for these layers, respectively. Although there is some differences in sample materials between the calculations and the experiments, the calculated C-Ga values for Si and W layers roughly agree with the experimental values for the magneto-optical disk TEM sample. This agreement firmly supports the present modeling of FIB-milled cross sections.