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Particle image velocimetry (PIV) was applied to estimate the velocity field on a weld pool surface with an oxide layer. A positive surface tension gradient drives an inward flow pattern from the boundary to centre of the weld pool, resulting in particles collecting on the centre line of the pool at the surface. Unsteady flow motions were observed by experimental visualisation of the particle flow, and the computed velocity field shows strong unsteady interfacial movement during inward flow. These unsteady flow motions are related to clockwise and counterclockwise vortices on the weld pool surface, which in turn affect both mass flow and energy transport. Both the qualitative flow motion and quantitative flow velocity are described in this work, which contributes to explaining the characteristic unsteady fluid flow in the weld pool, the measurements also provide experimental data for validation of numerical fluid flow models of the weld pool.