ZnO powder was synthesized via a hydrothermally assisted sol-gel method. Synthesis variables including Zn-precursor type, pH value, time, and temperature of hydrothermal treatment were analyzed using the Taguchi approach. The experimental procedures were defined based on the L9 array for four variables in three levels. The influences of those variables on the response parameters, i.e. crystallite size, crystallinity, band gap energy ($E_{g})$, and degradation constants ($k)$, were evaluated. XRD results and $E_{g}$ values showed that ZnO wurtzite appeared to be the only crystalline phase in the samples. Taguchi analysis predicted that the optimized conditions to achieve the highest photoactivity are as follows: Zn-precursor $=$ zinc acetate, pH $=$ 8, t $=$ 2 h, and T $=$ 150 $^{\circ}$C. The optimized sample was synthesized based on the mentioned conditions and characterized. The obtained results confirmed the prediction of the Taguchi method and the highest $k$ value was observed.