It is a reality that nowadays the demand for energy is increasing in the world, but at the same time conventional energy resources are becoming expensive, rare, and more pollutant. All these facts are leading the countries to focus on renewable energy sources, as they are cleaner and abundant. In this field Offshore Wind Power Plants (OWPPs) are becoming increasingly relevant in Europe, and worldwide mainly due to space limitations constraints (possibility of using larger wind turbines), the fact wind speeds are potentially higher and smoother at sea (leading to higher power generation), lower visual and noise impact than onshore farms, and finally because of the progressive saturation of propitious onshore sites. Currently, and because of the environmental and social legislation, OWPPs are forced to be constructed further from shore. There are three main factors to be covered when designing AC electric system of OWPPs: investment cost of components, system efficiency, and system reliability. The present project is focus on the first two key factors, and also considers a stochastic optimization of the electric system of an OWPP operation in order to minimize the investment and operational cost, and in this way try to get the best scenario obtaining the most of benefits. GAMS and MATLAB softwares have been used to implement the main program, obtaining a basic engineering tool to design and take decisions for the electric power system applied to real offshore wind farm.