The band gap opening of bilayer graphene with one side surface adsorption of F4-TCNQ is reported. F4-TCNQ doped bilayer graphene shows p-type semiconductor characteristics. With a F4-TCNQ concentration of 1.3 x 10(-10) mol/cm(2), the charge transfer between each F4-TCNQ molecule and graphene is 0.45e, and the built-in electric field, E-bi, between the graphene layers could reach 0.070 V/angstrom. The charge transfer and band gap opening of the F4-TCNQ-doped graphene can be further modulated by an externally applied electric field (E-ext). At 0.077 V/angstrom, the gap opening at the Dirac point (K), Delta E-K = 306 meV, and the band gap, E-g = 253 meV, are around 71% and 49% larger than those of the pristine bilayer under the same E-ext.