Uniaxial deformation of the conical conduction band of single-layer graphene is investigated on a SiC(0001) substrate vicinal to the [1100] direction using angle-resolved photoemission spectroscopy. The substrate surface consists of highly anisotropic terraces with sub-10 nm width in the [1100] direction as confirmed by scanning tunneling microscopy and atomic force microscopy. Graphene covers all the substrate surface including the substrate steps. The substrate provides a unidirectional potential for the graphene on that. The group velocities of the conduction band at K points are reduced in the direction parallel to the strong potential modulation while they are unchanged in the perpendicular direction.