Graft polymerization of isoprene onto styrene–butadiene rubber (SBR) was carried out in latex using p-menthane hydroperoxide and ferrous sulfate as an initiator. The effect of isoprene on vulcanization characteristics, mechanical, and thermal properties of isoprene-grafted SBR were investigated. The grafting of isoprene onto SBR was confirmed by attenuated total teflectance–Fourier transform infrared spectroscopy. The fatigue heat of the isoprene-grafted SBR mainly come from deformation and restructuring, while the smaller deformation of isoprene-grafted SBR will cause the heat lower. The results revealed that the glass transition temperature decreases with the grafting of isoprene and strengthen the Payne effect in grafted SBR.