Using a single crystal of a Cu-6 mass% Ag alloy aged at 723 K for various times, nano-sized Ag precipitates in a Cu matrix were observed by conventional and ultra high-voltage transmission electron microscopes. The nano-sized Ag precipitates with a radius of about 5 nm had a nearly spherical shape. The shape change from the nearly spherical shape to the {111} octahedral shape occurred with increasing the size of the Ag precipitates to 11 nm. Elastic states of the Ag precipitates were evaluated by observing moir, fringes formed between the Ag precipitates and the Cu matrix. The origin of the shape change of the nano-sized Ag precipitates was discussed by considering the sum of elastic strain energy and interfacial energy.