In this paper, UV-vis spectrum method and dynamic light scattering (DLS) methods were used to evaluate the stability of gamma-Al2O3 nanofluid. The results of DLS method indicate that zeta potential increases by increasing ultrasonic mixing time. After 150 min, zeta potential was 50 mV, which under these conditions the nanofluid was stable. The results also show that the size of the nanoclusters decreases with ultrasonic mixing time. The results obtained by UV-vis spectrum indicate that by increasing the ultrasonic mixing time, the value of maximum absorbency increases and after 30 min, it decreases gradually. After 150 min of ultrasonic mixing time, the maximum absorbency becomes constant. This mixing time can be considered as optimum mixing ultrasonic time.