Differential scanning calorimetry (DSC) was applied to natural rubber (NR) and NR/recycled natural rubber (RNR)(NR/RNR) blend to understand and optimize the state of cure, which is based on the value of the exothermic reaction obtained in process of vulcanization. Swelling and mechanical data were investigated with the increase of cure time from 15 to 35 minutes with an interval of 5 minutes and compared with the DSC enthalpy data. With the increasing cure time, cure enthalpy was decreased and a negative, while cure enthalpy was a positive value in 35-NR and 35-NR/RNR when cure time was over 35 minutes due to the reversion. Swell index (Si) was gradually decreased but increased slightly in the case of both NR and NR/RNR cured for 35 minutes. The study also indicated that cure enthalpy data was very precise and fast to predict the cure state of the NR and NR/RNR blend.