Offshore wind power technology is still in development, therefore there is a gap for new designs in order to reduce costs and increase the efficiency. The cable size is an important factor regarding the large investments required in an offshore wind farm. The main aim of this thesis is to optimize the cable sections for three different offshore wind farms, taking into account the costs of the cables and the costs of the energy losses. This optimization is performed through a multi-objective function and the optimum point of the Pareto front. Firstly, the fundamentals of the project are presented, such as a brief explanation of the parts concerning an offshore wind power plant, paying special attention to the cables as are the element of study; the electric design, focusing on the cable model and the power flow analysis; the Newton-Raphson method, since is the method applied for the optimization; and the cost models for the cables and energy losses due to being the objective of minimization. Secondly, the optimization is explained and further applied for cable sizing, and the preliminary work is introduced. Thirdly, the three cases of study are presented, with the reference and optimized design respectively, and a brief discussion of the results is made. Finally, the conclusions are drawn, firstly for the results of the case study and afterward for the hole project.