Electron spin resonance(ESR)spectroscopy was employed to elucidate the setting process of the glass-ionomer cement by use of manganese ions(Mn^<2+>)as a spin probe.The effects of powder liquid ratio, temperature, mixing time, additives of tannin fluoride and tartaric acid, and particle size of the powder on setting time were investigated.The glass-ionomer cement powder was mixed with cement liquid containing Mn^<2+>.The peak height of the highest field component of hyperfine splittings due to nuclear spin of Mn(I=5/2), which was proportional to the concentration of free Mn^<2+> was measured at 30-second intervals beginning 2.5 minutes after mixing.The setting time decreased with the increase in the powder liquid ratio, increase in the enviromental temperature, decrease in the mixing time, and decrease in the powder particle size.Although tannin fluoride had no essential effect on either setting process or setting time, the presence of tartaric acid increased the setting time.However, the setting time thus estimated by ESR without any load, was slightly longer than that estimated with a load of 454g according to the ADA No.9 specification.The findings obtained by use of 2, 2, 6, 6-tetramethylpiperidine(TEMPO)as a spin probe instead of Mn^<2+> were also reported.