In this article, a rectangular cross section breakwater in water of finite depth and infinite domain is studied using the separation of the variables method in regular, sinusoidal waves. Determining the radiation potentials in three degrees of freedom e.g. sway, heave and roll, added mass and damping coefficients of sway, heave and roll motions is obtained. Diffraction problem is solved according to linear wave theory and resulting forces on structure for three degrees of freedom is obtained. As far as validation of this study is concerned, results are compared with other researches. Having the diffraction potentials in each region, the transmission and reflection coefficients and drift force are obtained. Furthermore, a parametric study on the effects of water depths, floating breakwater sizes as well as wave number on transmission and reflection coefficients carried out. Some new achievement is concluded by this study. The results show that mean drift force on the breakwater increases as the ratio of width to height of the breakwater, increases in different water depths. Also, increasing the depth of water results in increasing of mean drift force, similarly. However, the effect of the width of the breakwater is more pronounced than the effect of the depth of water.