The sintering kinetics and mechanisms of micron-size and nanocrystalline titanium powders have been studied by means of dilatometry and sintering diagram models. An approximate particle-size range over which a transition in sintering mechanisms takes place has been identified. In the micron-size titanium powder, lattice diffusion from surface sources, and from sources on grain boundaries, are the operative mechanisms. The nanocrystalline titanium powder shows a continuous change in sintering mechanisms: (i) particle movement/dislocation movement at low temperatures, (ii) grain boundary diffusion up to moderate temperatures, and (iii) lattice diffusion and grain boundary rotation at higher temperatures.