Conversion degrees (CDs) of double bonds of 8 dental composites were measured by Fourier transform infrared spectroscope. Also, effects of disc-sampling method and film-sampling method on CD were compared. The results showed that CDs of 8 composites ranged from 45% to 75%. CDs of visible-light-cured (VLC) composites were higher than those of chemically-cured (CC) ones. In VLC composites, those with hybrid fillers had higher CDs than those with superfine fillers. In VLC composites, CDs obtained by disc-sampling method were higher than by film-sampling method; in CC composites, there were no differences between CDs obtained by both sampling methods.