Tetrapod-shaped ZnO nanocrystals were synthezised via thermal evaporation of Zn powder in air. The Zn powder was oxidized at 930 degrees C for 60 min. To investigate the optimal Zn/O molar ratio in such vapor phase synthesis, the Zn content was varied in range of 0.1-0.75 g. When the Zn content was 0.1 g, no nanocrystals were formed. Above 0.25 g Zn, growth of ZnO nanocrystals started without clear tetrapod like morphology. Beyond Zn content of 0.50 g, clear tetrapod-shaped ZnO nanocrystals were detected. These results establish that there is a certain required ratio of zinc and oxygen in vapor phase for generation of tetrapod-shaped ZnO nanocrystals. Such Zn:O ratio is also calculated theoretically from the ideal gas law. Clear tetrapod type ZnO nanocrystals prepared with Zn content of 0.5 g exhibited the highest intensity of the ultraviolet emission centered at 380 nm which also confirms the high crystalline quality of such ZnO nanocrystals. (C) 2012 Elsevier B.V. All rights reserved.