This paper describes a comparative study on the effect of electric current pulse (ECP) and the semi-sinusoidal current (SSC) on the microstructure of directionally solidified Al-4.5 wt% Cu alloy. The experimental results indicate that both ECP and SSC can make the convex solid/liquid interface flat, reduce the primary spacing and the depth of mushy zone, and change the interface morphology from cellular to planar (i.e. delay the planar/cellular transition). Under the same heating power, SSC produces a greater influence on the microstructure than ECP. Limited effect of the current direction on microstructure is also observed. Generally, current flowing from the liquid to the solid have a stronger effect on the microstructure than those from the solid to the liquid. Possible mechanisms by which the current altes the microstructure of Al-4.5 wt% Cu alloy under directional solidification conditions are suggested. (C) 2011 Elsevier B.V. All rights reserved.