Amorphization and radical formation from micro- and nano-sized particles of cystine in mechanochemical process were examined. Dry grinding was used to create amorphous cystine particles. Cystine samples after dry grinding were characterized using a scanning electron microscope, the Brunauer-Emmett-Teller method, powder X-ray diffraction, electron spin resonance (ESR), and Fourier transform infrared (FT-IR) spectroscopy. The kinetic characteristics of amorphization from nano-sized particles were assessed from the ball impact energy and force calculated using the distinct element method (DEM) simulations. The results showed that the kinetics of amorphization almost followed the Johnson-Mehl-Avrami equation using DEM simulations without adhesion. In addition, the amorphous progression exceeded the threshold value of the impact force. The radical species of cystine particles were identified by evaluating the substance-specific value from ESR spectra and the absorption intensity of FT-IR spectra. The results demonstrated that the radical produced by grinding was a thiyl radical (RS center dot) because the peaks at 452 cm(-1) in the spectra resulted from the characteristic absorption frequency of disulfide (S-S). The radical formation was closely related to the amorphization of cystine particles. (C) 2016 Elsevier B.V. All rights reserved.