Diffusion coefficients are calculated from the concentration distribution in diffusion couples composed of solid Al and liquid Al-Cu alloys and of solid Al and Al-Cu alloys halfway solidified. Diffusion coefficients in liquid alloys are found to be almost independent of Cu concentration up to 20%. Diffusion coefficients are expressed in accordance with liquid diffusion. The solid phase boundary is almost stationary during diffusion of solid Al and Al- Cu alloy halfway solidified.