In this investigation, an mm-wave RoF transmission link with dual beat-mode optical source is investigated. We demonstrate a 170 GHz band millimeter-wave source using an external dual-mode-locked Fabry-Perot laser diode (FP-LD) with a 1.38 nm mode-spacing. Two distributed feedback (DFB) lasers are used to inject the corresponding modes of FP-LD to generate dual-beat-mode output. The proposed dual-mode laser is not only stable, but also has single-longitudinal-mode (SLM) output characteristics. Moreover, we also investigate the relationship between two external-injected wavelengths for the dual-wavelength output.