An actively mode-locked laser with a high average power at a high repetition rate was designed and constructed. A harmonic mode locking technique and a regenerative feedback loop method were applied to realize an actively mode-locked Nd:YLF laser. A Numerical simulation was performed to obtain a stably operative condition of the laser. In this thesis, we present the design and construction of a regeneratively third harmonic mode-locked 1 GHz Nd:YLF lasers operating with a long term stability. A Nd:YLF laser has been actively mode-locked at the third-order harmonic cavity frequency using a phase modulator. This laser cavity consisted of an active medium with a diode pumping, a MgO:$LiNbO_3$ phase modulator, an etalon, and a folded laser cavity including a folding mirror and an output coupler. The regenerative feedback loop consisted of a high-speed photo-diode on a linear translator as a phase shifter, a high gain radio frequency (RF) amplifier, and a phase modulator. A 1 GHz RF signal to drive the phase modulator was obtained from a 1 GHz pulse train, without using a RF synthesizer as a driver of an active mode locker, which solved the synchronization problem in the phase modulator by automatically tracking any change in the cavity length. The numerical simulation on the regeneratively harmonic mode-locked laser showed that an insertion of an etalon could provide a stable laser operation by avoiding a mode competition problem. We achieved a stable 1 GHz pulse train with a pulse duration of 6.3 ps, and an average output power of 250 mW at 1053 nm. This laser has a good potential for many applications, such as a photocathode laser to generate high current photoelectrons useful for high power free electron lasers or particle accelerators.