A quantum key distribution (QKD) scheme using binary-modulated coherent states is proposed in this paper. Compared with Gaussian sources of QKD using Gaussian-modulated coherent states, the binary sources of the proposed scheme are the simplest sources, and the binary modulation is the most usual modulation format in the digital optical-fiber communication. The security of the proposed schemeagainst beam splitter attack is analyzed using Shannon information theory. The analytical expression of the secret information rate is given in terms of modulation-demodulation parameters and channel parameters. The quantum noise of coherent states guarantees the security of the proposed scheme.