Two different types of Nd:YAG laser oscillators are used simultaneously in order to increase the weld penetration depth. A rectangularly modulated CW beam emitted from a CW-pumped laser oscillator together with a pulse beam emitted from a pulse-pumped oscillator is irradiated onto stainless steel (304). This paper investigates the contribution of a pulsed beam to welding characteristics in two-laser-beam welding. A pulsed beam with high peak power is effectively combined with a modulated CW beam for adjusting the power density in order to obtain deep penetration. The plume is extinct during modulated CW beam irradiation after the emission of a pulsed beam under deep penetration conditions. On the basis of this phenomenon, keyhole behaviour could be considered and it is recognized that a pulsed beam with high peak power would play a significant role in enlarging the keyhole. A strong correlation between the penetration depth and the plume extinction time is apparent. Therefore, the keyhole might be temporarily but sufficiently enlarged and penetration might be consequently deepened by a pulsed beam with high peak power.