Semi-interpenetrating networks (semi-IPNs) were prepared from natural rubber (NR) and polystyrene (PS) by the sequential method. In these semi-IPNs the NR phase was crosslinked while the PS phase was uncrosslinked. Different initiating systems such as dicumyl peroxide(DCP), benzoyl peroxide (BPO), and the azobisisobutyronitrile (AIBN) system were used for polymerizing the PS phase. The blend ratio was varied by controlling the swelling of NR in the styrene monomer. The mechanical properties of the semi-IPNs, namely, density, tensile strength, tear strength, elongation at break, tension set, tensile set, impact strength, and hardness, were determined. The morphology of different IPNs was studied using scanning electron microscopy. A compact morphology with a homogeneous phase distribution was observed in the semi-IPNs. The properties of the semi-TPN do not change much with the initiating system. However, in most cases, the DCP initiating system showed slightly superior performance. The tensile and tear-strength values of the IPNs were found to increase with increasing plastomer content. The crosslink density of the semi-IPNs also increased with increase in the polystyrene content. The experimental values were compared with theoretical models such as series, parallel, Halpin Tsai, Coran, Takayanaki, Kerner, and Kunori. The tensile and tear-fracture surfaces were examined using a scanning electron microscope. The fracture patterns were correlated with the strength and nature of the failure.