We are reporting here on an inexpensive and facile fabrication method for ZnO nanorod arrays by hydrothermal growth at low temperature (90°C). In our experiment, ZnO nanostructures were grown on glass substrate using an equimolar (0.1M) aqueous solution of Zn(NO3)2•6H2O (zinc nitrate hexahydrate) and C6H12N4 (HMTA) as precursors solution, and using ammonia solution to controlling the pH levels. It enable easily obtained arrayed ZnO nanorods on substrate, and nanowires which grown on nanorod arrays were identified after about 1 month in the air. The growth process of nanorods and the formation mechanism of nanowires were investigated.