The present paper studies the mechanism of interface formation in the particle growth in an undercooled alloy melt with a far-field uniform flow. Using the method of matched asymptotic expansion, we obtain an approximate solution for particle growth and determine the pattern formation of crystal-melt interface. The analytical results show that the far-field uniform flow has significant effect on particle growth and the mechanism of the pattern formation of crystal-melt interface is similar to that of the convective pure melt. The drag effect of the far-field uniform flow results in the higher local growth rate in the upstream front, where the flow is incoming and the lower local growth rate in the downstream rear, where the flow is outgoing. Compared with the situation in the pure melt, the solute in the undercooled alloy melt further facilitates the growth of the particle. The critical radius for instability decreases as the far-field uniform flow increases. (C) 2010 Elsevier B.V. All rights reserved.