In order to investigate the multiplication of equiaxed grains by the forced convection, in-situ observation using a transparent organic substance have been made. The emission of some spherical particles from the columnar zone was observed by a fiber scope installed in an ingot case. These particles are originally the secondary dendrite arms and are detached by the fluctuations of thermal and/or solutal fields and/or mechanical breaking. Then the particles fell down in the liquid along with the natural convection. This event has taken place approximately 30 s before the well- developed equiaxed grains started to form. Therefore, these particles developed to equiaxed dendrites floating around in the liquid.
A physical model for formation of equiaxed grains based on the observation has been proposed. The experimental results on the change in multiplication rate with time could be reasonably explained by this physical model.