Cross sections of the patterns fabricated in (100) GaAs by 100‐keV gallium focused ion beam have been studied using a scanning electron microscope (SEM). The probe size of the ion beam is 0.1–0.15 &mgr;m at the current of 100 pA. The etched depth becomes saturated at the high dose region (about 5.0×10−6C/cm) because of the redeposition effect. The pattern profile becomes asymmetric if it is made up of several adjacent lines perpendicular to the beam scanning direction due to the redeposition effect and the increase of sputtering yield for each scan, which is caused by the change of ion beam incident angle. These effects can be eliminated by the use of multiwriting method.