Directional solidification of the undercooled melt which combines melt undercooling with conventional directional solidification is a new method of obtaining directional structure. This method is used to realize the directional solidification of the undercooled Ag70.3Cu29.7 melt. The experiment is performed in two steps: The undercooled melt is obtained by glass fluxing, and the above specimen is remelted and excited with Ga-In-Sn coolant. By this method, directional solidification dendrites of Ag70.3Cu29.7 alloy are achieved successfully. The columnar dendrites are straight and fine. The primary arm spacing is 21µm in average for a 65K undercooling, as compared to 38µm for a 32K undercooling. This confirms that higher undercooling promotes finer dendritic microstructures.