Targeted therapy with tamoxifen is an effective method to treat breast cancer. This method requires competent drug delivery vehicles to ensure successful therapeutic practices. The stable adsorption between the drug and delivery vehicle is one of the essential components. Using first-principles calculations, the adsorption behaviors of tamoxifen on reduced graphene and graphene oxide were studied based on density functional theory. The results indicated that tamoxifen was weakly adsorbed on pristine graphene, while it was relatively strongly adsorbed on reduced graphene oxides. Our results concluded that among the systems of reduced graphene oxide with an oxygen concentration of 0%, 3.125%, and 12.5%, graphene sheets with oxygen were potential candidates for tamoxifen delivery vehicles for breast cancer targeted therapy, and graphene oxide with an oxygen concentration of 12.5% was the most promising one compared to other carbon-based vehicles.