In the present study, we report the fabrication of polybutadiene rubber (PBR)/graphene nanosheet (GNS) nanocomposites using in situ polymerization. For the catalyst system, we chose neodymium versatate (NdV3) as the catalyst, triethylaluminum as the cocatalyst or activator, and ethylaluminum sesquichloride as the chloride donor in the presence of GNSs. The effects of GNSs on the activity of the catalyst, chain microstructure, structural, morphological, and thermal properties of the nanocomposites have been investigated. The morphology, structural properties, and the state of dispersion of graphene nanoplatelets in the PBR matrix have been characterized by X-ray diffraction spectroscopy, scanning electron microscopy, and transmission electron microscopy which showed dispersion of the individual layers (highly exfoliated) of graphene in the PBR matrix. Thermal properties of nanocomposites have been studied by differential scanning calorimetry (DSC) and thermogravimetric analysis. DSC results indicated that the addition of small amounts of well-dispersed graphene significantly increased the glass transition temperature (T-g) owing to restricted motion of rubber chains physically adsorbed or wrapped to graphene surfaces because of pi-pi and CH-pi interactions. The incorporation of GNSs enhanced the thermal stability and char yield of the nanocomposites. Based on these results, in situ polymerization mainly paves the way to prepare graphene-based nanocomposites of rubbers with good properties and high performance.