The temperature effects on water's protective and assistant role for uracil have been investigated at a wide range of temperature. It indicates that the water molecule assisting proton transfer at low temperature could prevent proton transfer at high temperature and the water molecule preventing proton transfer at low temperature could assist proton transfer at high temperature. Further research reveals that the proton transfer of uracil is governed by entropy change. Why the role of water molecule reverse at different temperature was explained. A relationship between strong/weak hydrogen bond and proton transfer could be suggested. (c) 2005 Elsevier B.V. All rights reserved.