In　this　paper,　given　the　beamforming　vector　of　confidential　messages　and　artificial　noise　(AN)　projection　matrix　and　total　power　constraint,　a　power　allocation　(PA)　strategy　of　maximizing　secrecy　rate　(Max-SR)　is　proposed　for　secure　directional　modulation　networks.　By　the　method　of　Lagrange　multiplier,　the　analytic　expression　of　the　proposed　PA　strategy　is　derived.　To　confirm　the　benefit　from　the　MaxSR-based　PA　strategy,　we　take　the　null-space　projection　beamforming　scheme　as　an　example　and　derive　its　closed-form　expression　of　optimal　PA　strategy.　From　simulation　results,　we　find　the　following　facts:　in　the　medium　and　high　signal-to-noise-ratio　(SNR)　regions,　compared　with　three　typical　PA　parameters　such　beta　=　0.1,　0.5,　and　0.9,　the　optimal　PA　shows　a　substantial　SR　performance　gain　with　maximum　gain　percent　up　to　more　than　60%.　In　addition,　as　the　PA　factor　increases　from　0　to　1,　the　achievable　SR　increases　accordingly　in　the　low　SNR　region,　whereas　it　first　increases　and　then　decreases　in　the　medium　and　high　SNR　regions,　where　the　SR　can　be　approximately　viewed　as　a　convex　function　of　the　PA　factor.　Finally,　as　the　number　of　antennas　increases,　the　optimal　PA　factor　becomes　large　and　tends　to　one　in　the　medium　and　high　SNR　regions.　In　other　words,　the　contribution　of　AN　to　SR　can　be　trivial　in　such　a　situation.