High-aspect-ratio Ag nano- and microplates have been produced via an inhomogeneous system, i.e., coordination-polymer fine crystals of silver oxalate, Ag-2(C2O4), suspended in an aqueous solution containing poly(vinylpyrrolidone) as a reducing and surface-protecting agent. The Ag plates are nucleated and grown exclusively on side-surfaces along the longitudinal axis (c axis) of the rod-like Ag-2(C2O4) crystals, in which the position and direction of the C2O42- are suitably ordered to generate the morphology-controlled Ag plates.