A systematic study of the extraction probability of surface-produced H - ions from the ion source has been done with H- and neutral Monte-Carlo transport modeling. Without the effects of the flow velocity of background H+ ions and neutrals, the extraction probability (E.P.) depends strongly on the strength of the magnetic filter field B MF for the low gas pressure condition. On the other hand, the E.P. does not depend so much on BMF for the high gas pressure condition. Neutral transport modeling shows that H2 neutral possibly has a relatively high velocity (∼1km/s). The effect on the E.P., however, is not significant even for the high gas pressure regime.