Mechanism of vortex formation in continuous casting moulds was investigated using a full scale water model. It was found that vortices occurring near the SEN was due to interaction between the two surface streams flowing inwards. Biased flow as such does not generate vortices in the mould, but increases its depth and frequency. Vortex depth increases with increasing casting speed. Vortex formation in the mould can be reduced by diminishing biased flow and optimising the SEN design. When casting wide slab at a shallow SEN immersion depth, vortex formation takes place near the narrow faces.