This study is to observe the change of temperature and relative humidity for various vegetables in vacuum precooling system. The materials for experiments were the lettuce, chinese cabbage, pak choi and cabbage. The experimental apparatus was constructed of vacuum chamber, vapor/water separator, water tank, pumps ejecting and cooling water circulation, refrigerator unit, cooling coil for water cooling, Hygrometer and Data logger measuring of the temperature change. The experiments were operated in 20torr and recorded every 3 minutes. It was found that the cooling temperature and speed of vegetables are depending on the percentage of its water content. The more water contains, the faster cooling speed and the lower cooling temperature.