This research studies the morphological changes of water in a vacuum cooling system in the course of vacuum chamber pressure step-down. The experiment result shows that steam adhered to vacuum chamber due to the atmospheric pressure evaporates as the chamber pressure decreases and produces a great amount of smoke after the vacuum pumping system is initiated for 20 s. After 115.2 s, the liquid water in the vessel begins to produce tiny bubbles and then large bubbles at 121.2 s, and boils at 126.7 s. After the rapid boiling is over, the liquid water enters the freezing stage at 181.5 s. When the vacuum pumping system is stopped at 480 s, the liquid water inside the vessel is frozen into solid ice which is consisted of two layers, irregular porous layer on the top and dense layer on the bottom. The cooling rate of liquid water in this experiment is between 0.33-0.38 degrees C/s, and 0.04-0.07 degrees C/s during the freezing process. (c) 2005 Elsevier Ltd. All rights reserved.