This study was undertaken to examine an effect of eugenol on polymerization of methyl methacrylate (MMA) by benzoyl peroxide (BPO) and 2,2'-azobisisobutyronitrile (AIBN) in the presence of eugenol. The induction period and initial rate of polymerization (IRP) were determined from polymerization curves of MMA using differential scanning calorimetry (DSC). The induction period increased with increasing concentration of eugenol in both BPO and AIBN systems. The IRP decreased as the concentration of eugenol in both BPO and AIBN systems. The IRP decreased as the concentration of eugenol increased. Its decreasing rate in the BPO system was higher than that in the AIBN system. In the BPO system, the IRP was reduced to zero at 0.5 mol% of eugenol. Eugenol in the BPO system was an efficient retarder, although the decrease in IRP was relatively small, below 0.05 mol%. Eugenol was an inhibitor even at high concentration in the AIBN system. The suppressible effect of eugenol appears to be due to the interaction between free radicals from BPO and eugenol.