In the present work, graphene was synthesized by catalytic reduction of carbon dioxide (dry ice) in presence of magnesium flame supported by zinc and nickel metal respectively. The use of Nickel resulted in the improvement of graphene yield. The multiwall carbon nanotubes (MWCNTs) were synthesized from pongamia oil in CVD setup. The synthesized graphene and MWCNTs were characterized by different characterization techniques such as scanning electron microscopy, transmission electron microscopy, Raman spectra and X‐ray diffraction study. Thus synthesized graphene and MWCNTs have been used as conductive filler in polystyrene (PS)/graphene and PS/MWCNT nanocomposites. PS nanocomposites were prepared by solution blending method using toluene as a solvent. The properties of PS/graphene nanocomposites were compared with those of PS/MWCNT nanocomposites and the graphene based nanocomposites showed improved results. Interestingly, PS/graphene nanocomposite showed good electrical conductivity (about ∼7.07 S/cm) and these also showed electromagnetic interference shielding effectiveness (–31 dB) with the loading of 10 wt% graphene in PS matrix. Thermal properties of PS/graphene nanocomposite were also studied and were found to be good. POLYM. COMPOS., 39:E1041–E1051, 2018. © 2017 Society of Plastics Engineers