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This paper concerns the dynamic variation in plume velocity and keyhole size during the laser welding process of 5083 Al-alloy. The keyhole size and plume velocity were measured from high-speed images through imaging process methods and the particle image velocimetry (PIV) method. The correlation between the oscillation in plume velocity and keyhole size was discussed based on the pressure balance inside the keyhole. Results show that the keyhole keeps opening and collapsing during the welding process. During the time the keyhole is open, the size of the keyhole continuously oscillates until it collapses. Plume velocity increases to a maximum at the initial stage of keyhole's opening, and then varies in synchronisation with the oscillation of the keyhole size. It is considered that local evaporation occurs on the humps of the keyhole front wall, which results in the oscillation in plume velocity. The oscillation in plume velocity then leads to the variation in the pressure balance in the keyhole and drives the keyhole to expand or shrink accordingly. [ABSTRACT FROM AUTHOR]