This work discusses different roles of nano-silica in the kinetics of peroxide vulcanization of ethylene propylene diene (EPDM) rubber for the first time. Initially, it was shown that silica does not participate in the vulcanization mechanism by chemical bonding to other intermediates. Non-isothermal differential scanning calorimeter revealed that the enthalpy of vulcanization, proportional to the crosslinking reaction, goes through a minimum as silica loading increases. Therefore, silica has two roles affecting kinetics of peroxide vulcanization of EPDM: physical effect on the rate of crosslinking reaction through reduced dynamics of rubber chains and catalytic effect on the decomposition of peroxide. In the low-temperature isothermal vulcanization, the enthalpy of vulcanization decreased monotonically, indicating that the physical effect is dominant. The extent of physical effect was correlated to the immobilized bound rubber resulting from the filler-polymer and filler-filler interactions. However, at high-temperature isothermal or non-isothermal vulcanization, the catalytic effect surmounts the physical one. (C) 2017 Elsevier Ltd. All rights reserved.