The results of investigations on synthesis of single-layer graphene on nickel substrate in the flame of premixed benzene-oxygen mixture at the pressure of 45Torr with and without application of electric potential direct current of positive and negative polarity are presented. It is stated that at low pressure in benzene-oxygen flame, single-layer graphene is synthesized with minimum defects I-D/I-G=0.27. The temperature of substrate 900 degrees C and exposure time 0.5-1min are sufficient for synthesis of single-layer graphene. It is shown that application of electric potential to the substrate influences synthesis of graphene in the flame. It is stated that at negative and positive potentials on a nickel substrate, formation of single-layer graphene is observed, at the value of negative potential U=-2V on a nickel substrate, one can observe formation of defectless single-layer graphene (I-D/I-G approximate to 0).