Zinc aluminate (ZnAl2O4) particles have been synthesized by the hydrothermal method using NH3 . H2O as a pH adjustment mineralizer. Experimental results showed that ZnAl2O4 particle size was dependent on the precursor pH, and could be controlled through pH adjustment. It was 5.5, 11.5, and 27 nm when the precursor pH was 8.2, 9.3, and 10.5, respectively. On the other hand, the particle size distribution changed broader with increase in pH. These differences were attributable to the different NH3 . H2O function. NH3 . H2O was mainly used as a base at lower pH (<9.0), while its complex function predominated at higher one (>9.5). From thermodynamic viewpoint, the rate-limiting steps were dissolution of AI(OH)3 and gamma-AIO(OH) to AI(OH)4 at lower and higher pH, respectively. The newly formed gamma-AIO(OH) with high reactivity was the critical factor in the synthesis of bimodal particles. Higher temperature treatment of gamma-AIO(OH) could decrease the reactivity, and could be used as an aluminum source for synthesis uniform ZnAl2O4 particles.