Graphene nanosheets/polystyrene (GNS/PS) composites were prepared by in-situ free radical solution polymerization. The graphene nanosheets were dispersed well in the polymer matrices due to the covalent interface between the inorganic and polymer phases. The effect of the amount of the initiator added on the thermal stability and electrical conductivity of the composites were compared. Thermogravimetric analysis (TGA) and gel permeation chromatography (GPC) analyses of the polystyrene grafted graphene nanosheets (GNS-PS) samples showed that more polystyrene chains with a relatively smaller molecular weight had been grafted onto the graphene nanosheets via free radical addition with more initiator added to the solution polymerization.