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The glass transition temperature in amorphous region of insulation paper is one of the most important characteristics for its thermal stability. In order to study the microscopic mechanism of the glass transition process for transformer insulation paper which may provide some information for thermal aging, molecular dynamic simulations has been performed on two micro-structural models: pure amorphous cellulose and amorphous cellulose with water. Using the method of specific volume versus temperature curve, the glass transition temperatures of pure cellulose and cellulose-water were determined as 448K, 418K respectively. Simulation results also show that during the glass transition process, both the chain movement (characterized by mean square displacement, end-to-end distance, and mean square bend) and mechanical properties of cellulose are changing significantly. The addition of water also reduces the mechanical strength of cellulose and increases its ductility and plasticity slightly. In addition, Fox and Flory's free volume theory was also used to explain the nature of glass transition which shows a sudden change of free volume between 400 K and 450 K that provides more space for chain movement. The water molecules that immersed in amorphous region of insulation paper can disrupt hydrogen bonds between cellulose chains, which lead to a significant reduction in glass transition temperature.