Abstract Despite a large number of approaches developed for wireless sensor network (WSN) localization, there are still many unsolved problems in this area. The challenges to be addressed are both in analyzing characterizations of the localizable WSNs and designing efficient localization algorithms under a variety of conditions. In this paper we first draw on powerful results from graph rigidity theory and combinatorial theory, revealing that the combination of distance constraint and bearing constraint leads to necessary and su fficient condition for unique localization. This enlightens our proposing an anchor-free and computationally simple ad hoc localization algorithm for WSNs. A novel combination of distance and direction estimation technique is introduced to detect and estimate ranges between neighbors. Using this information we construct unidirectional local coordinate systems to avoid the reflection ambiguity. Such local maps then converge to form a global network wide coordinate system using a transformation matrix [T], which finally leads to node absolute positions. Simulation results have shown that our algorithm achieves high accuracy without using any error refining schemes. Keywords Unique localization analysis, Anchor-free, Localization algorithm, Wireless sensor network fficient condition for unique localization. This enlightens our proposing an anchor-free and computationally simple ad hoc localization algorithm for WSNs. A novel combination of distance and direction estimation technique is introduced to detect and estimate ranges between neighbors. Using this information we construct unidirectional local coordinate systems to avoid the reflection ambiguity. Such local maps then converge to form a global network wide coordinate system using a transformation matrix [T], which finally leads to node absolute positions. Simulation results have shown that our algorithm achieves high accuracy without using any error refining schemes. Keywords Unique localization analysis, Anchor-free, Localization algorithm, Wireless sensor network