We describe a sub 100-mW H.264 MP@L4.1 integerpel motion estimation processor core for low power video encoder. It supports macro block adaptive frame field (MBAFF) encoding and bidirectional prediction for a resolution of 1920×1080 pixels at 30fps. The proposed processor features a novel hierarchical algorithm, reconfigurable ring-connected systolic array architecture and segmentation-free, rectangle-access search window buffer. The hierarchical algorithm consists of a fine search and a coarse search. A complementary recursive cross search is newly introduced in the coarse search. The fine search is adaptively carried out, based on an image analysis result obtained by the coarse search. The proposed systolic array architecture minimizes the amount of transferred data, and lowers computation cycles for the coarse and fine searches. In addition, we propose a novel search window buffer SRAM that has instantaneous accessibility to a rectangular area with arbitrary location. The processor core has been designed with a 90nm CMOS design rule. Core size is 2.5×2.5mm². One core supports one-reference-frame and dissipates 48mW at 1V. Two core configuration consumes 96mW for two-reference-frame search.