Ga focused-ion-beam (FIB)-assisted etching of single-crystal diamond and thin film diamond in XeF2 was studied. The etch yield in FIB-assisted etching of diamond in XeF2 is enhanced some six times over the physical sputtering yield. In the crystal orientation dependence of the etch yield in FIB-assisted etching, the (100) face produced the highest etch yield of the three faces-(100), (110), and (111). Thin film diamond produces the lowest etch yield. A diamond field emitter with a tip radius of less than 100 nm was obtained using Ga FIB spot exposure.