Thermally induced evolution of phase transformations is a basic physical-chemical process in the dissociation of gas hydrate in sediment (GHS). Heat transfer leads to the weakening of the bed soil and the simultaneous establishment of a time varying stress field accompanied by seepage of fluids and deformation of the soil. As a consequence, ground failure could occur causing engineering damage or/and environmental disaster. This paper presents a simplified analysis of the thermal process by assuming that thermal conduction can be decoupled from the flow and deformation process. It is further assumed that phase transformations take place instantaneously. Analytical and numerical results are given for several examples of simplified geometry. Experiments using Tetra-hydro-furan hydrate sediments were carried out in our laboratory to check the theory. By comparison, the theoretical, numerical and experimental results on the evolution of dissociation fronts and temperature in the sediment are found to be in good agreement. Natural gas hydrate (NGH) is a crystalline solid composed mainly of methane gas and water molecules, and is stabilized at high pressure and low temperature [1,2]. NGH is extensively distributed in sediments of oceans, continental margins, permafrost zones and deep lakes [3,4]. When the phase equilibrium is destabilized, NGH will dissociate into gas and water. Generally, 1 m 3 of NGH may release 164 m 3 methane gas and 0.8 m 3 of water at 1 atm at normal temperature. Then a large excess pore pressure will result in a strength decrease of the sediment if the released gas diffuses slowly [5,6].During exploitation of gas hydrate or gas and oil in the deep sea, high-temperature pipes will pass through GHS and make the temperature of GHS increase. Accordingly, NGH in sediment will dissociate and the GHS and cap-rock become unstable. The dissociation zone extends with time and