The effects of fluid flow on the purification of aluminum were studied. As the revolution rate(N) increased, the size of columnar grain decreased gradually. The concentration of solidified crystal was decreased with increasing distance from chill and revolution rate(N). Distribution boundary layer thickness(δ) was calculated from the solute distribution obtained in solid experimentally and by use of BPS equation. The value of δ changed from about 60㎛ at N value of 27rpm to about 15㎛ at N value of 1000rpm. From this result, high purification was obtained by decreasing the diffusion boundary layer under forced convection.