We propose and demonstrate a low-cost 40-GHz optical millimeter-wave (mm-wave) generation scheme constructed by the utilization of a frequency-tripling directly modulated Fabry-Perot (FP) laser. The scheme requires neither a strong optical injection power nor an additional optical filter to select the desired optical sidebands. Error-free transmission of the generated 40.2-GHz optical mm-wave carrying 2.5-Gb/s signal over a 20-km standard single-mode fiber (SSMF) has been experimentally demonstrated. In addition, a performance comparison with the conventional case of an injection-locked FP laser directly driven with a 40.2-GHz RF signal is also simulated, which shows that the presented scheme with highly reduced bandwidth requirement is suitable for mm-wave radio-over-fiber (RoF) systems.