The coupled oscillations of partially filled reinforced concrete (R/C) rectangular tanks with fluid and soil interactions were studied in this study. Two different rectangular tanks having flat bottoms with 0.5 m and 1 in wall thicknesses are considered. It is assumed that the tanks are situated on six different soil types defined due to well-known earthquake codes. Fluid-Rectangular Tank-Soil/Foundation systems are modeled with the finite element (FE) technique and analyzed using full-transient analysis. In these models, a displacement based fluid FE approximation implemented for taking into account fluid-structure interaction by means of general purpose computer code ANSYS and 3D-Solid FE model with viscous boundary is used to consider soil/foundation interaction effects. Finally, deviations of the sloshing responses, displacement of the tank walls orthogonal to the excitation, and reaction forces obtained from the results are discussed at the end of the study. It is shown that when the soil-structure interaction is considered, the sloshing amplitude of the fluid and displacements of the wall are not affected on this type of tanks, however, the fluid-structure interaction are fairly effective on the seismic behavior of the tank wall orthogonal to the excitation.