Sintering and grain growth of nano-crystalline undoped ZnO has been studied in detail over a wide range of temperature and holding time. Below 800°C, sintering of over 70% theoretical density is not observed, irrespective of particle size. At 900°C for 6h, the nano-crystalline sample sinters to 99% of theoretical density whereas the density for as received sample is 93% of theoretical density. However, at 1300°C or higher, the densification is found to be much faster and after a few hours becomes independent of holding time. Grain growth studies reveal a similar feature of attaining saturation over holding time. The average saturated grain size is found to be ∼1.5 and ∼2.2μm at 800 and 900°C, respectively, while at 1300°C or higher, it is in between 12 and 13μm.