We demonstrate a stable and strong n-type doping method to tune the electrical properties of graphene via vapor phase chemical doping with various high-molecular-weight ethylene amines. The resulting carrier concentration after doping with pentaethylenehexamine (PEHA) is as high as -1.01 × 10(13) cm(-2), which reduces the sheet resistance of graphene by up to ∼400% compared to pristine graphene. Our study suggests that the branched structure of the dopant molecules is another important factor that determines the actual doping degree of graphene.