The mechanical performance of full-interpenetrating polymer networks (IPNs) based on natural rubber/polystyrene (NR/PS) system has been studied in detail. The IPNs were prepared using different initiators viz., benzoyl peroxide (BPO), azo-bis-isobutyronitrile (AIBN), and dicumyl peroxide (DCP) and the effect of initiating systems on the properties was studied. It was observed that in all cases the IPNs initiated using DCP showed a superior property compared to BPO and AIBN initiated ones. The crosslink density of the IPN was varied by varying the percent of divinyl benzene (crosslinking agent of PS) and the effect of crosslink density on the properties has been studied. The stress-strain behaviour, tensile strength, Young's modulus, elongation at break, tear strength, tension set and tensile set were determined. The effects of strain rate on tensile properties were analysed. The studies on the morphology using scanning electron microscopy showed an increase in phase mixing on increasing the PS content and PS crosslinking. However, high level of PS content and PS crosslinking lead to a decrease in phase mixing. The morphology studies using TEM revealed the interesting fact that NR/PS IPN system was nano-structured. The fracture surfaces of tensile and tear specimens were studied using scanning electron microscopy, to get a clear picture of the mechanism of failure.