Abstract: The adjusting law of withdrawal speed that made the solid/liquid interface position locate near the insulating plate stably was obtained bv simulating the whole mode-withdrawal type directional solidification process in which the interface was controlled by a computer. The control system based on single-board computer TP801B was built to control the directional solidification process on line using the law obtained above. Experimental results for an Al-4.5%wt Cu alloy show that the solid/liquid interface was based to insulating plate quickly and located near it stably throught the whole process. The results also show that this technique was effective in raising the stability, it was also effective in improving the microstructure of directionally solidified castings.