Modification of starch is being intensively studied and modified starch is used for applications in synthetic polymers. The essential force for this activity is due to both biodegradability and a renewability resource of starch. In many studies starch and its derivatives are used as biodegradable fillers for different synthetic polymers. Unfortunately, the mechanical properties of thermoplastic polymer/starch blends are very poor due to the incompatibility and hydrophilic nature of starch. For this purpose, graft polymerization of thermoplastic polymers onto starch is one of the best methods for preparing composites of starch with synthetic polymers. In the present research, graft copolymer of starch-polystyrene (PS-g-starch) was prepared by an internal mixer as reactor and using benzoyl peroxide as thermal initiator at various temperatures (110-130?C). To achieve this samples, first a predetermined pre-mixture containing native starch, styrene monomer and thermal initiator was prepared at the same time by a mechanical mixer. The mixture was feed into the internal mixer device. In order to achieve the highest amount of grafted polystyrene on starch, some factors such as starch/styrene monomer weight percent ratio, amount of thermal initiator, reaction time, the internal mixer speed of rotor and reaction temperature were change. To evaluate grafting percentage from extraction analysis the synthetized homopolymer was extracted by Soxhlet apparatus using toluene as solvent. Weight percent of homopolymer was calculated by weighting the obtained samples. The homopolymer weight percent (PS wt. %) was found to be 40.42%. Acetic acid and prcoloric acid were used to isolate grafted part from the starch compound to calculate grafting percentage. The highest grafted polystyrene on starch particles was 9.60%. To confirm the grafting reaction the Transfer Infrared Spectroscopy (FTIR) and the Scanning Electronic Microscope ( SEM ) of the extracted sample were used. Transfer Infrared Spectroscopy spectrum showed new peaks in range of 700, 765, 1553 and 3058 cm -1 for C-H in aromatic rings of polystyrene and the peaks present in interval of 1700-2000 cm -1 for C=C present in aromatic ring before and after Soxhlet extraction. The scanning electronic microscope micro graphs of the samples before Soxhlet extraction showed the synthesized polystyrene covers starch particles surface and after Soxhlet extraction grafted polystyrene remained on starch particles surfaces. These result showed that the polystyrene was grafted on the surface of starch particles. To investigate of thermal stability of samples, after and before Soxhlet extraction the thermal gravimetric analysis (TGA) was used. Result indicated that the sample before Soxhlet extraction showed two step thermal degradation. The first step occurred in 275-355 ?C indicates starch part degradation and the second step occurred in 355-465 ?Crepresenting the polystyrene degradation. Polystyrene increase the thermal stability of native starch. The sample after Soxhlet extraction showed one step thermal degradation. This result is because of extraction homopolymer part from sample. To evaluate the process ability of compounds, the melt flow index was determined. Result showed that with increasing of grafted percentage to 9.60%, the process ability of samples was increasing too. Keywords: Starch, polystyrene, graft copolymer, internal mixer.