In this research, mesoporous silica (MPSiO₂) nanoparticles were dispersed in n-octadecane as an organic phase change material (PCM) in order to produce a novel composite for thermal storage. Stable PCMs containing 1 wt.%, 3 wt.% and 5 wt.% MPSiO₂ nanoparticles (PCM/MPSiO₂) were fabricated by dispersing MPSiO₂ in PCM. MPSiO₂ particles were investigated by SEM and TEM techniques, which showed high order of porosity and spherical particles of ca. 300 nm. The thermal conductivity in both solid and liquid phases was measured by transient plane source (TPS) technique in the temperature range of 5–55 °C. A maximum thermal conductivity enhancement of 5% for 3 wt.% MPSiO₂ at 5 °C, and 6% for 5 wt.% MPSiO₂ at 55 °C was experimentally obtained. Moreover, it was observed that enhancement in thermal conductivity is non-monotonic in solid phase with increasing MPSiO₂ particle loading. The viscosity results showed that for mass fractions of nanoparticles greater than 3% in liquid PCM, the behavior of liquid is non-Newtonian. Also, the viscosity of PCM containing MPSiO₂ nanoparticles was measured to be increased up to 60% compared to the liquid PCM for 5 wt.% MPSiO₂ at 35 °C.