It is essential to tailor the electronic properties of graphene in order to apply graphene films for use in electrodes. Here we report the modification of the electronic properties of single layer chemical vapor deposition (CVD) grown graphene by molecular doping without degrading its transparency and electrical properties. Raman spectroscopy and transport measurements revealed that p-toluenesulfonic acid (PTSA) imposes n-doping on single layer CVD grown graphene. The shift of G and 2D peak wave numbers and the intensity ratio of D and G peaks are analyzed as a function of reaction time. In the gate voltage dependent resistivity measurement, it is found that the maximum resistivity corresponding to the Dirac point is shifted toward a more negative gate voltage with increasing reaction time, indicating an n-type doping effect. We have also made single layer graphene p–n junctions by chemical doping and investigated the current–voltage characteristics at the p–n junction.