The effect of buoyancy-driven convection on the steady state dendritic growth in an undercooled binary alloy is studied.For the case of the moderate modified Grashof number,the uniformly valid asymptotic solution in the entire region of space is obtained by means of the matched asymptotic expansion method.The analytical results show that the buoyancydriven convection has a significant effect on the needle-like interface of dendritic growth.Due to the buoyancy-driven convection,the needle-like interface shape of the crystal is changed.When the Peclet number that is not affected by the buoyant flow is less than a certain critical value,the interface shape of the dendrite becomes thinner as the Grashof number increases;when it is larger than the critical value,the interface shape becomes fatter as the Grashof number increases.In the undercooled binary alloy the morphology number plays an active role in the interface shape and leads to the buoyancy effect that is different from the situation for the pure melt.The smaller the morphology number is,the more significant change the interface shape has.As the Peclet number further increases,the effect of buoyancy on the interface diminishes eventually. The effect of buoyancy-driven convection on the steady state dendritic growth in an undercooled binary alloy is studied. For the case of the moderate modified Grash of number, the smaller the effective asymptotic solution in the entire region of space is obtained by means of the matched asymptotic expansion method. analytical results show that the buoyancy driven convection has a significant effect on the needle-like interface of dendritic growth. Due to the buoyancy-driven convection, the needle-like interface shape of the crystal is changed. is not affected by the buoyant flow is less than a certain critical value, the interface shape of the dendrite becomes thinner as the Grashof number increases; when it is larger than the critical value, the interface shape becomes fatter as the Grashof number increases. the undercooled binary alloy the morphology number plays an active role in the interface shape and leads to the buoyancy effect that is different from the situatiion on for the pure melt. The smaller the morphology number is, the more significant change the interface shape has. As the Peclet number further increases, the effect of buoyancy on the interface diminishes eventually.