Abstract: Directional solidification experiments were carried out using the analogue casting system of NH₄Cl-H₂O solution by cooling it from below. The transient velocities of fluid flow within the micro-channel of mushy layer were measured by using observation facility and tracer coal particles of 30 μm in diameter. It turned out that during the solidification process, the mean solid fraction of mushy layer descended rapidly at first, so the mushy layer got more permeable and natural convection was more likely to occur. When the mean solid fraction was down to 0.42, and closed to the smallest value of 0.38, and the equivalent Reynolds number exceeded the critical value, about 247, plume convection with associated micro-channel occurred in the mush. With the expanding of channel width, the solution of liquid region, whose temperature was higher than that of mushy layer, flowed into the micro-channel. The convection within the channel was found to consist of upward flow and downward flow. The counter flow advected heat away and solute which was favorable to solidification, so that solution within the channel recrystallized.