The dependence of mode-locked oscillation of homogeneous lasers on the modulation frequency of locking signal is studied. An experiment using a He-Ne 3.39 μm laser and applying intracavity phase (FM) modulation was performed on the frequency range of 40 MHz to 1.20 GHz. The oscillation bandwidth was observed to be nearly proportion to √ƒ m and ƒ m (ƒ m: modulation frequency) for the modulation frequency range of ƒ m<Δƒ and ƒ m>Δƒ (Δƒ: linewidth), respectively. The possibility to generate ultra-short pulses by using larger modulation frequency than the linewidth is also discussed. For 1.20 GHz modulation frequency, the pulsewidth was estimated to be about 70 ps from the observed power spectrum, while the reciprocal of the linewidth was about 3 ns.