The thermal behaviour of a horizontal cylindrical storage tank has been investigated both experimentally and numerically. Four sets of experiments have been carried out where cold water is injected into the bottom of the tank with three different initial thermal fields. The first one is the tank with initial thermal stratification with bottom temperature the same as the inflow temperature. The second set is the tank with the initial thermal stratification, the bottom being at a relatively higher temperature than the inflow temperature. The third set is an initially heated isothermal tank and the fourth is the same as the first set of experiments except that the straight tube inlet nozzle is replaced by a 30 degrees downward bent divergent conical tube. The above experiments show that better thermal stratification can be obtained using the divergent conical tube as the inlet nozzle due to the diffusion effect of the nozzle. Also a slight improvement in the tank performance has been achieved in the second set of experiments when the initial bottom temperature of the tank is higher than the injected cold water temperature. To check the accuracy of the experimental results two different types of one-dimensional numerical models, namely Turbulent Mixing Model and Displacement Mixing Model have been developed and the results are compared with the experiments. This comparison indicates that the numerical results are in good agreement with the experiments especially at the top of the tank.