Nanometer-scale GaAs ring structure is self-assembly realized by droplet epitaxy in a lattice-matched system. By changing the As-4 flux intensity during the crystallization of Ga droplets into GaAs, balance between the crystallization inside and at the edge of the droplets is changed, resulting in the shape control from dot to ring. The ring structure exhibits clear photoluminescence emission up to room temperature. Droplet Epitaxy is a promising growth method not only for quantum dots but also for quantum rings, with high structural and optical qualities in lattice-matched systems. (© 2005 Elsevier B.V. All rights reserved.