ZnO rod arrays have been grown on glass substrates with different pH by using a simple hydrothermal method at 95 degrees C without using a seed layer. Zinc nitrate hexahydrate ((Zn(NO3)(2).6H(2)O) was used as source of zinc ion, while hexamethylene tetrammine (HMT, C6H12N4) solution served as the complexing agent. Ammonia (NH4OH) and acetic acid (CH3COOH) were used to achieve the desired pH value. No deposition is observed for the solution having pH values of 4 and 10. The effect of pH of precursor solution on structural, morphological and optical properties of ZnO rod arrays was investigated. XRD results indicate that the ZnO rods arrays are growing along  direction with a high consistent oriental:ion perpendicular to the substrate. SEM images reveal that increase in the pH value from 5 to 9 leads to decrease in the size of rods grown on substrate. The optical measurements show that there is no significant change in the band gap of rod arrays with increasing the pH of the starting solution.