Finding the core members of a virtual community is an important problem in community analysis. Here we presented an simulated annealing algorithm to solve this problem by optimizing the user interests concentration ratio in user groups. As an example, we test this algorithm on a virtual community site and evaluate its results using human "gold standard" method. Categories and Subject Descriptors H.3.3 [Information Systems]: Information Search and Retrieval; H.3.5 [Information Systems]: Online Information Services; J.4 [Computer Applications]: Social and Behaviorial Sciences General Terms Algorithms, Experimentation,Human Factors Keywords Virtual community, core members finding, simulated annealing