Focused ion beam (FIB) technology is becoming increasingly important for submicron device processing. However, the generation of ripples on the substrate surface bombarded with an FIB at off-normal ion incidences will become a problem in three-dimensional fabrication using an FIB. Therefore, we have examined the 30 keV Ga-FIB milling of a single-crystal Si wafer and a diamond-like carbon (DLC) film deposited on a Si wafer by FIB chemical vapor deposition (CVD). After FIB milling, samples inclined at 45° were observed by scanning ion microscopy (SIM) to determine the surface morphology. As a result, we confirmed the 30 keV Ga-FIB milling conditions resulting in the generation of ripples on a DLC film and a single-crystal Si wafer. Then, we obtained ripple-free milling conditions for these materials.