To clarify the effect of centrifugal force in unidirectionally solidified metals, Al-0.7wt% Cu alloy system was selected as test material. Centrifugal force was added from 1G(G: territorial acceleration=9.8m/s²) to 20G ; test pieces were quenched at about 50% solid fraction. Planar interface morphology became unstable with increasing centrifugal force, but at the centrifugal force is about 15G, the interface morphology of this alloy became stable again. In order to understand this interface stability under the high centrifugal force apparent diffusion coefficient were measured and also flow modes over solid-liquid interface were analyzed by Favier Method. The Be´rnard cell formed by centrifugal force enhances the heat flow from top of test piece to solid-liquid interface but it and gravitational segregation also restrict enhancement of diffusion in some extant. We could infer qualitatively that solid-liquid interface stability was due to this enhancement of heat flux by centrifugal force at solid-liquid interface.