In this paper, a novel 18-element asymmetric double-it equivalent circuit model for on-chip spiral inductors is proposed with corresponding direct parameter extraction. Meanwhile, the asymmetric characteristics of spiral inductors are analyzed by introducing a geometry-based optimization factor. To validate the proposed model and asymmetric analysis, two series of circular inductors with different number of turns and radii were fabricated and model parameters were extracted from S-parameters measurement up to 20 GHz. The results show that this modeling methodology can achieve high accuracy extraction while offering simplification to previously reported methods. (C) 2009 Elsevier Ltd. All rights reserved.