In this work, studies are carried out to understand the crosslinking reaction of epoxidized natural rubber (50 mol% epoxy, ENR-50) by metal ion namely ferric ion (Fe³⁺, FeCl₃, ferric chloride). It is found that a small amount of FeCl₃ can cure ENR to a considerable extent. A direct interaction of the ferric ion with the epoxy group as well as internal polymerization enable the ENR to be cured in an efficient manner. It was also found that with the increased concentration of FeCl₃, the crosslinking density of the matrix increased and therefore, the ENR offers higher mechanical properties (i.e., modulus and tensile strength). In addition, the glass transition temperature (t_g) of ENR vulcanizate is increased with increasing concentration of FeCl₃. Moreover, the thermal degradation temperature (T_d) of the ENR-FeCl₃ compound was shifted toward higher temperature as increasing concentration FeCl₃.