Characteristics of ion beam assisted etching (IBAE) for SiO2 have been investigated to reveal'a possibility for maskless etching using focused ion beams. The ion beam assisted etching was done by bombarding 50 keV unfocused ion beams in XeF2 atmosphere and effect of various etching parameter on etching characteristics have been investigated. These are the effects of XeF2 gas pressure, bombarding ion species, bombarding angle and H2 addition, etc. Significant enhancements up to 100 times larger than physical sputtering were achieved. The selectivity of SiO2 to Si could be tailored to specific requirements from 0.1 to 6 by changing the gas mixing ratio. The etching rate was approximately proportional to the energy deposition rate bombarded by ion beam on surface. Carbon contamination on surface after etching were improved by the introduction of XeF2 gas.