Colloidal nanocrystals have long been used to study the dependence of phase stability and transitions on size. Both structural phase stability and phase transitions change dramatically in the nanometre size regime where the surface plays a significant role in determining the overall energetics of the system. We investigate the solid-solid phase transformation of crystallization in amorphous GeTe nanoparticles. We report a colloidal synthetic route to amorphous GeTe nanoparticles. Using in situ X-ray diffraction while heating, we observe the crystallization of the nanoparticles and find a dramatic increase of the crystallization temperature of over 150 °C above the bulk value. Using size-selected nanoparticle films, we show that the crystallization temperature depends strongly on the particle size. In addition, we measure the electrical resistance of nanoparticle films and observe over 5 orders of magnitude lower resistance for the crystalline film compared to the amorphous film. Finally, we discuss the implications of the size-dependence of crystallization in the context of both understanding the behavior of phase stability in the nanosize regime and applications to phase change memory devices. © 2010 The Royal Society of Chemistry.