SimRank is a classic measure of the similarities of nodes in a graph. Given a node u in graph G = (V , E), a single-source SimRank query returns the SimRank similarities s(u, ) between node u and each node ∈ V . This type of queries has numerous applications in web search and social networks analysis, such as link prediction, web mining, and spam detection. Existing methods for single-source SimRank queries, however, incur query cost at least linear to the number of nodes n, which renders them inapplicable for real-time and interactive analysis. This paper proposes PRSim, an algorithm that exploits the structure of graphs to efficiently answer single-source SimRank queries. PRSim uses an index of size O(m), where m is the number of edges in the graph, and guarantees a query time that depends on the reverse PageRank distribution of the input graph. In particular, we prove that PRSim runs in sub-linear time if the degree distribution of the input * Work partly done at Beijing Key Laboratory of Big Data Management and Analysis Method, Renmin University of China. † Ji-Rong Wen is the corresponding author.Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org.