In the past decade, the bag-of-feature model has established itself as the state-of-the-art method in various visual classification tasks. Despite its simplicity and high performance, it normally works as a black box and the classification rule is not transparent to users. However, to better understand the classification process, it is favorable to look into the black box to see how an image is recognized. To fill this gap, we developed a tool called Restricted Support Region Set (RSRS) Detection which can be utilized to visualize the image regions that are critical to the classification decision. More specifically, we define the Restricted Support Region Set for a given image as such a set of size-restricted and non-overlapped regions that if any one of them is removed the image will be wrongly classified. Focusing on the state-of-the-art bag-of-feature classification system, we developed an efficient RSRS detection algorithm and discussed its applications. We showed that it can be used to identify the limitation of a classifier, predict its failure mode, discover the classification rules and reveal the database bias. Moreover, as experimentally demonstrated, this tool also enables common users to efficiently tune the classifier by removing the inappropriate support regions, which can lead to a better generalization performance. © 2012 IEEE.