Abstarct: One of the major difficulties related to crude oil storage tank is settling heavy particles and forming accumulated sludge at the bottom of the tank. Accumulated sludge which is mostly composed of asphaltene and wax particles, causes to decrease the tank storage capacity and corrosion. Due to high density of asphaltene and wax particles, these particles settle due to gravity force. In the present work, a numerical study has been carried out to study for the effect of solar irradiation on sludge forming of the crude oil storage tanks. We use Angstrom model for calculating solar irradiation for every month of year and then predicting mean crude oil temperature in tank. An experimental and numerical study has been carried out to investigate sludge formation at bottom of an in-use crude oil storage tank. The numerical method has been developed to determine amount of monthly sludge formation. Firstly, a thermodynamic model which is baxsed on equality of fugacity for each component in both the solid and liquid phases has been developed to predict wax precipitation. The amount of asphaltene precipitation is determined by experiments. Secondly, Stoke's Law, Fick's law of diffusion and Einstein's law of diffusion have been employed to detect the critical particle size and critical particle molecular weight. Critical particle molecular weight which is the size of smallest particle that settles under the force of gravity. Then by knowing a particle size distribution, a fraction of the incoming asphaltene and wax particles which settle in the tank has been calculated. Finally, the numerical value of sludge height has been compared with similar measured value and good agreement has been encountered. In experiment investigation, Khark 28th crude oil storage tank is selected as a case study and sludge height has been measured every month for more than a year. The numerical and measured value of sludge height shows 350 mm growth during a year. This corresponds to 30926 barrels of crude oil and means 30926×75$= 2319450$ financial lost for just one crude oil storage tank.