In directional solidification, compositional convection can be driven by an unstable density gradient in the melt. In this paper convective instabilities in liquid and mushy layers during solidification of a horizontal binary melt are analyzed by using the propagation theory. The self-similar stability equations are used to examine the boundarymode and mushy-layer-mode of convection. The effects of velocity conditions at the liquid-mush interface on the onset of convection are discussed. The critical Darcy-Rayleigh number for the convection in the mushy layer decreases with increasing the temperature of a cooled boundary.