Dispersion and aggregation behavior of nanoparticles in aquatic environment may be affected by pH, salinity, and dissolved organic matter, which would change its ecological risk. Effects of time, power and temperature on the alumina nanoparticles (nano-Al₂O₃) ultrasonic dispersion in water were discussed. Al₂O₃ had a best ultrasonic dispersion for 30 min at 105 W and 30 °C. The concentration of Al₂O₃ could be measured by ultraviolet (UV) spectrophotometer, and the method was efficient and accurate. Furthermore, the sedimentation rate of Al₂O₃ was related to pH, salinity, and its concentration in the artificial seawater. When pH was 7.31, approaching the isoelectric point of Al₂O₃, they aggregated and settled fastest. Settlement coefficient () of Al₂O₃ increased by 3 and 2.7 times while the salinity and its concentration increased. The sedimentation rate was higher in natural seawater than that in artificial seawater. All results indicated that nano-Al₂O₃ would be removed in aquatic environment.