The electronic structure of an electron-acceptor molecule, tetracyanoethylene (TCNE), on graphene was investigated using the first-principles method based on density functional theory. It was theoretically demonstrated that a p-type graphene can be obtained via charge transfer between an organic molecule and graphene. Both the carrier concentration and band gap at the Dirac point can be controlled by coverage of organic molecules. The spin split and partially filled pi* orbitals of the TCNE anion radical induce spin density in the graphene layer. Sur-face modification of graphene by organic molecules could be a simple and effective method to control the electronic structure of graphene over a wide range.