The aim of this work is to improve the performance of a Savonius Vertical Axis Wind Turbine (VAWT) by sizing a suitable rotor guide plates configuration, or what is called a turbine's casing, using Computational Fluid Dynamics (CFD) technique. Starting from a proposed baseline casing design, a 2-D parametric optimization process was followed, where several design parameters pertaining to the casing's geometry were assigned and optimized. Owing to the limitations of the 2-D numerical simulations, the optimized casing dimensions were extracted and used to carry out 3-D numerical investigations. The turbine with the optimized casing readily performed better than the caseless one, especially at lower Tip Speed Ratios (TSR). In addition, the obtained results showed that there exist no universal optimal values for the casing dimensions that maintain peak turbine performance at all TSRs. Thus, a clear trend relating all the optimized casing dimensions to the TSR was established. (C) 2019 Elsevier Ltd. All rights reserved.