fine-grained image categorization is a challenging task aiming at distinguishing objects belonging to the same basic-level category, e.g., leaf or mushroom. it is a useful technique that can be applied for species recognition, face verification, and so on. most of the existing methods either have difficulties to detect discriminative object components automatically, or suffer from the limited amount of training data in each sub-category. to solve these problems, this paper proposes a new fine-grained image categorization model. the key is a dense graph mining algorithm that hierarchically localizes discriminative object parts in each image. more specifically, to mimic the human hierarchical perception mechanism, a superpixel pyramid is generated for each image. thereby, graphlets from each layer are constructed to seamlessly capture object components. intuitively, graphlets representative to each super-/sub-category is densely distributed in their feature space. thus, a dense graph mining algorithm is developed to discover graphlets representative to each super-/sub-category. finally, the discovered graphlets from pairwise images are integrated into an image kernel for fine-grained recognition. theoretically, the learned kernel can generalize several state-of-the-art image kernels. experiments on nine image sets demonstrate the advantage of our method. moreover, the discovered graphlets from each sub-category accurately capture those tiny discriminative object components, e.g., bird claws, heads, and bodies.