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The study is devoted to the processing of hydroxyapatite (HAp) nanopowder to develop fully dense nanostructured bioceramics by pressureless sintering. The sintering behavior of stoichiometric HAp prepared by hydrothermal processing was investigated by nonisothermal, two-step, and conventional sintering. By low-temperature two-step sintering (TSS), at 900°C and 850°C, with appropriate dwell time, dense bioceramics without final-stage grain growth and average grain size of 75 nm was obtained. A concept of master sintering curve was applied, enabling control of sintering process, estimation of effective activation energy for sintering of HAp nanopowder, and qualitative understanding of sintering mechanisms. According to estimated activation energy of 412.6 kJ/mol, low sintering temperature and particles' microstructure as the dominant sintering mechanism we proposed diffuse-viscous flow controlled by grain boundary diffusion. HAp nanoparticles comprising of different interior and boundary regions as ordered/disordered microstructure are found to be of an advantage for low-temperature sintering.