n-tetracosane (C24H50), a phase change material (PCM) that undergoes a phase transition at 50.6 °C, was successfully encapsulated by silica (SiO2) as shell material in order to prevent it from leakage in this study (n-tetracosane@SiO2). Additionally, silver nanoparticles (Ag NPs) with various sizes (20, 40, and 80 nm) having excellent thermal conductivity were loaded on the capsules to improve its thermal conductivity (n-tetracosane@SiO2@Ag NPs). It has been found that the encapsulation capacity, the thermal conductivity of the capsule, and the degree of n-tetracosane leakage affect the thermal energy storage capacity of the encapsulated n-tetracosane@SiO2@Ag NPs. As the size of Ag NPs loaded on the encapsulated n-tetracosane@SiO2 increased the thermal conductivity of capsules increased, especially when the Ag NPs of 80 nm was loaded the thermal conductivity was enhanced more than three and two times compared to pure n-tetracosane and n-tetracosane@SiO2 capsule, respectively. The maximum melting and crystallization points in n-tetracosane@SiO2@Ag NPs (80 nm) were 51.05 and 43.27 °C, respectively, temperatures higher than those of pure n-tetracosane. The encapsulated n-tetracosane@- SiO2@Ag NPs (80 nm) exhibited a fusing and freezing latent heat capacities of 170.51 and 169.07 J/g, respectively, an energy-storage efficiency of 63.41%, and a thermal storage capacity of 100%. Moreover, even with repeated fusing and freezing over 100 cycles, there was no significant change in the thermal energy storage capacity, and the shape of the capsules was maintained as it was. These results indicate that the encapsulated n-tetracosane@SiO2@Ag NPs have good thermal stability, which could be used as form.stabilized phase change materials for thermal energy storage.