Fibrillated cellulose can be used as a gelation agent to thicken and improve phase stability of sodium sulfate decahydrate (SSD, Na2SO4·10H2O), a promising salt hydrate for storing thermal energy. It is expected that dissolved SSD influences the thickening action by changing the colloid interactions between the cellulose fibrils. The fibrillation degree and surface charge of the cellulose are hypothesized to control the thickening effect. Phase stability of dissolved SSD was evaluated by sedimentation. Viscoelastic behaviors were characterized to understand microstructural and thickening mechanism created by fibrillated cellulose. Effect of fibrillated cellulose on heat release of SSD was measured during phase change. Dissolved SSD reduced the electrostatic repulsion between the cellulose fibrils by compressing the electric double layer due to charge screening. This resulted in denser aggregates for the mechanically fibrillated cellulose, assisted by increased attraction force. Viscosity and storage modulus of SSD increased and stable phase was formed without sedimentation. Higher fibrillation degree of the mechanically produced cellulose eliminated phase separation due to the increased specific surface area. The phase stabilization of SSD resulted in higher and longer heat release caused by bonding between water and anhydrous sodium sulfate.