As one of mechanochemical effects, free radicals effect during ultrafine comminution of konjac glucomannan was studied. The techniques of ultraweak inescence and electron spin resonance were used to study the formation and changes of free radicals during ultrafine comminution of konjac glucomannan. Results showed that plentiful free radicals were produced during the ultrafine comminutionon. Within the examined range, the concentration of free radicals increased with prolonging the comminuting time. But the free radicals were unstable and tended to quenching after the comminution. This research provided the theoretical basis for mechanochemical modification of konjac glucomannan.