Nanomaterials in different sizes can be separated by an ultrasonic-assisted phase transfer method. A two-phase polar-non-polar liquid system composed of alcohol (polar, C-1-C-3 or their mixture), alkane (non-polar, C-5-C-10 or their mixture) and DI water is used in this method. The separation is achieved via the surface tension of the interface and the dynamic equilibrium established between nanomaterials of different weights. Different nanostructures can be separated to exist in layers of different polarity or in the interface throughout the sonication process, with the small-sized nanostructures staying in the upper layer and the large-sized one in the lower layer and interface. (C) 2008 Elsevier B.V. All rights reserved.