In recent years, many significant research efforts have been devoted to voltage security margins which show how close the current operating point of a power system is to a voltage collapse point as assessment of voltage security. In this paper we propose a technique based on the SOM-based fuzzy systems for voltage security margin estimation. The SOM-based fuzzy systems use the Kohonen’s self-organizing feature map (SOM) algorithm, not only for its vector quantization feature, but also for its topological property. The vector quantization feature of feature maps is used to search a good supply of most representative cluster centers. Then the topology-preserving feature is fully utilized to select a set of most influential rules so as to contribute to the computation of system outputs. The proposed technique was tested on 1604 simulated data randomly generated from operating conditions on the IEEE 30-bus system to indicate its high efficiency.