Formation and evolution details of a two-phase coupled microstructure in AISI 304 stainless steel are studied by quenching method during directional solidification. Results show that the coupled growth microstructure, which is composed of thin lathlike ferrite (d) and austenite (g), crystallizes first in the form of colony from the melt. As solidification develops, the retained liquid transforms into austenite gradually. On cooling, solid-state transformation from ferrite to austenite results in the disappearance of part thinner ferrites and the final two-phase coupled microstructure is formed after the solid-state transformation. The formation mechanism of the two-phase coupled microstructure is analyzed based on the nucleation and constitutional undercooling criterion (NCU) before steady-state growth of each phase is reached.