Abstract: Accurate knowledge of the autonomous system (AS) commercial relationships will contribute to research Internet structure and network behaviors intensively. Usually the AS relationships are obtained only by inference. Several algorithms on AS relationships inference are summarized and their advantages and disadvantages are discussed. A new metric is defined, which represents the transmission capability of an AS node based on its routing behaviors. The metric is more suitable to describe the level of AS nodes on transmission capability than the node degree. Based on the metric, a quick inference algorithm is presented which infers AS relationships by comparing the level of nodes. The experiment results show this algorithm has good consistency with Dimitropolous’s algorithm on the inference results, and the basic AS relationships can be inferred quickly and efficiently by this algorithm.