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An ohmic contact of graphene/MoS2 heterostructure is determined by using ultraviolet photoelectron spectroscopy (UPS). Since graphene shows a great potential to replace metal contact, a direct comparison of Cr/Au contact and graphene contact on the MoS2 thin film transistor (TFT) is made. Different from metal contacts, the work function of graphene can be modulated. As a result, the subthreshold swing can be improved. And when Vg<VFB, the intrinsic graphene changes into p-type, so graphene contact can achieve lower off current by lowering the Fermi level. To further improve the performance of MoS2 TFT, a new method using graphene contact first and MoS2 layer last process that can avoid PMMA residue and high processing temperature is applied. MoS2 TFT using this method shows on/off current ratio up to 6×106 order of magnitude, high mobility of 116 cm2/V-sec, and subthreshold swing of only 0.515 V/dec.