We have investigated the focused-ion-beam (FIB) lithographic, mechanism of Se75Ge25 resist exhibiting dual-type resist characteristics according to development methods. That is the Ga-FIB exposed Se75Ge25 films act as positive and negative type resists for wet-etching development using a solution of HNO3:HCl:H2O (1:1:3) and dry development by CF4 reactive-ion etching (RIE), respectively. With increasing incident energy, both sensitivity S and contrast Y are enhanced in the case of wet development, while both are deteriorated in the case of dry development. Considering the deposited energy density and implanted ion-concentration distributions obtained by our Monte Carlo simulation, FIB lithography for wet development depends primarily on the deposited energy and for dry development is dominated by the implanted ions themselves. For a 580 Angstrom thick Se75Ge25 resist exposed by 30 keV Ga+ FIB and developed by CF4 RIE for 5 s under the gas pressure of 200 mTorr, S and Y are about 1.7 X 10(15) ions/cm(2) and 4.44, respectively. In this case the ion concentration exceeds approximately 3.7 x 10(20) ions/cm(3), Ga-exposed region is estimated to have a resistance against CF4-reactive gas. A 0.125 mu m line pattern, with very small proximity effect, has been successfully fabricated by RIE development after 40 keV FIB exposure with a diameter of 0.120 mu m.