Nanocrystals ZnO are prepared by chemical precipitation.Crystal structure and ceramoc microstructure of powder samples are examined by XRD and TEM.The mean grain size of the materials was-calculated by Debye Scherrer method.The dependence of the mean grain size on reaction concentration is discussed.The results indicate that well-crystallized nano-ZnO is obtained with annealing temperature at 350℃.There exists a critical concentration which makes the size of nano-ZnO smallest.The other purpose of the present paper is to study the luminescence properties of the obtained sample.In particular the mechanism behind the visible luminescence has also been discussed in this paper.It should be said that the mechanism behind the visible luminescence is still a question of debate.Some authors attributed oxygen vacancies to the recombination centers.Other workers hypothesized that Zn vacancies were the centers responsible for green luminescence.The results obtained in the present paper support the point of view that Zn vacancies were the centers responsible for green luminescence and show that two phonon is involved in the emission processes.