We show that boron-doped epitaxial graphene can be successfully grown by thermal decomposition of a boron carbide thin film, which can also be epitaxially grown on a silicon carbide substrate. The interfaces of B4C on SiC and graphene on B4C had a fixed orientation relation, having a local stable structure with no dangling bonds. The first carbon layer on B4C acts as a buffer layer, and the overlaying carbon layers are graphene. Graphene on B4C was highly boron doped, and the hole concentration could be controlled over a wide range of 2 x 10(13) to 2 x 10(15) cm(-2). Highly boron-doped graphene exhibited a spin-glass behavior, which suggests the presence of local antiferromagnetic ordering in the spin-frustration system. Thermal decomposition of carbides holds the promise of being a technique to obtain a new class of wafer-scale functional epitaxial graphene for various applications.