Turbulent flow in a square duct after the expansion was investigated in detail. It was seen that the developing behavior of this flow is very different from that of the flow in straight square ducts. Especially, the secondaray flow in a square duct remains very weak and does not develop far downstream, at least through x/Dh = 40, unlike that in a straight square duct. This can be explained by the fact that many streamwise vortices, rather than a vortex pair in the corner region, develop from the distorted vortex loop caused by the free shear of the jet-like flow at the expansion. The results also indicaite that the turbulent intensities u prime, v prime, and w prime do not almost change from x/Dh = 20, at least to x/Dh = 40 (where Dh is the hydraulic diameter).