Y-shaped junctions and rings made of carbon nanotubes are proposed as possible nanostructures exhibiting quantum interference mechanisms in the electronic transport properties. The stability of the Y-structures are investigated by Monte Carlo simulations. The structures are quite stable for temperatures below 700 K. At higher temperatures, one founds a coalescence process between the two arms composing the symmetric Y-shaped rings and a formation of a localized defect in one of the arms of the asymmetric Y-shaped rings. Local electronic properties of a variety of metallic and semiconducting junction and rings are calculated by adopting a single-band tight-binding model. Remarkable changes in the electrical conductance are found by varying the length of one of the junction's arms. Interesting physical properties are investigated mainly concerned with interference processes manipulated by the sizes of the nanostructured materials. (C) 2004 Elsevier B.V. All rights reserved.