Thermal storage system with flat plate solar collector is performed in Faculty of Engineering, Menoufia University, Shebin El-Kom, Egypt, at Latitude of 30.56 degrees N and Longitude of 31.01 degrees E. The thermal storage system consists of insulated water tank, recirculation pump and flat plate solar collector to collect the solar energy and store it as a sensible energy. The measurements have been conducted from August 2015 to August 2016. The results showed that, the storage tank temperature is attained to about 80 degrees C in summer and 55 degrees C in winter. The average stored energy is attained maximum to 16.11 MJ/day in summer, and minimum to 10.26 MJ/day in autumn. The average yearly stored energy reached about 13.69 MJ/day. Zinc Oxide nanoparticles with average diameter of 23 nm have been added to the tap water to produce the nanofluid with volume fractions of 0.05% and 0.1%. The stored energy is increased by 3.36% and 7.78% for volume fraction of 0.05% and 0.1%. Also the daily efficiency is increased by 4.81% and 6.57%, for volume fraction of 0.05% and 0.1% compared to the previous state without nanoparticles.