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In this paper, we propose an effective method for detecting linear up-sampling in images. Our method includes three parts. First, we present an algorithm, Resampling Matrix Construction, that automatically derives the resampling matrix for any given resampling factor of fraction. Second, we show an algorithm, Zeroing Mask Derivation, that constructs a zeroing mask for the resampling produced by the proposed algorithm. Last, we propose an algorithm, up-sampling detection (UD), that detects the up-sampling in images using the zeroing masks in a specific order. Compared with existing methods, the proposed method for UD can not only effectively detect resampling by any fraction but can also recover the up-sampling fraction. Preliminary experimental results show that the proposed method is indeed effective.