When a single-layer graphene (SLG) is on SiO2 substrates, the charge exchange at their interface results in a dipole, which direction strongly depends on the contact potential difference between the SLG and the substrates. Due to the longer experimental charge screening length of SLG than its thickness, the charge redistribution imposes effective p or n doping to SLG films. The substrate-dependent doping of SLG films is further confirmed by Raman and electrical measurements. Also, the unique electronic structures of SLG films make them sensitive to the doping rather than effective gating from the SiO2 substrates.