This study aims to evaluate the potential of Al2O3-H2O nanofluids as a new phase change material for the thermal energy storage of cooling systems. Different mass fractions of nanofluids were prepared through adding Al2O3 nanoparticles and sodium dodecylbenzenesulfonate into water solution at 1 h of ultrasonic vibration. Measurement of particle size and zeta potential of nanofluids shows that Al2O3 nanoparticles have good dispersion in water, but Al2O3-H2O nanofluids with high mass fraction will make nanoparticles easier to aggregate. The thermal response test shows the addition of Al2O3 nanoparticles remarkably decreases the supercooling degree of water, advances the beginning freezing time and reduces the total freezing time. In order to visually observe the freezing process, an infrared imaging experimental system was built. The photographs suggest that the freezing rate of nanofluids is enhanced. Only adding 0.2 wt% Al2O3 nanoparticles, the total freezing time of Al2O3-H2O nanofluids can be reduced by 20.5%. (C) 2008 Elsevier B.V. All rights reserved.