The reduction of crystalline size and amorphization of graphite and diamond during agate and stainless ball-milling are investigated by Raman spectroscopy. The ultimate crystalline size of graphite, estimated by the Raman intensity ratio, of 2.5 nm for the agate ball-mill is smaller than that of 3.5 nm for the stainless ball-mill, while the milling time to reach the ultimate size for the former is about 10 times larger than for the latter, indicating more stability of the nanocrystalline graphite. After reaching the ultimate crystalline size, a significant broadening of the Raman spectra, which indicates the completion of amorphization, is detected only for the stainless ball-milled graphite at ∼500 h of milling. Also the increase rate of the Raman peakwidth for the stainless ball-milled graphite before amorphization is higher than that for the agate ball-milled graphite, indicating a larger introduction of disorder from the start of milling. Amorphization of diamond is also observed for the stainless ball-mill. The difference in the results between the agate and the stainless ball-mill is discussed in terms of the effect of impurity mixed from the milling apparatus on the stability of nanocrystalline carbon materials.