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Image search plays an important role in our daily life. Given a query, the image search engine is to retrieve images related to it. However, different queries have different search difficulty levels. For some queries, they are easy to be retrieved (the search engine can return very good search results). While for others, they are difficult (the search results are very unsatisfactory). Thus, it is desirable to identify those “difficult” queries in order to handle them properly. Query difficulty prediction (QDP) is an attempt to predict the quality of the search result for a query over a given collection. QDP problem has been investigated for many years in text document retrieval, and its importance has been recognized in the information retrieval (IR) community. However, little effort has been conducted on the image query difficulty prediction problem for image search. Compared with QDP in document retrieval, QDP in image search is more challenging due to the noise of textual features and the well-known semantic gap of visual features. This paper aims to investigate the QDP problem in Web image search. A novel method is proposed to automatically predict the quality of image search results for an arbitrary query. This model is built based on a set of valuable features that are designed by exploring the visual characteristic of images in the search results. The experiments on two real image search datasets demonstrate the effectiveness of the proposed query difficulty prediction method. Two applications, including optimal image search engine selection and search results merging, are presented to show the promising applicability of QDP.