Tidal energy is one of the major sources of renewable energy. To accelerate the development of tidal energy, improved designs of Tidal Current Turbine (TCT) are necessary. The effect of tower on performance and wake of TCT is investigated using Computational Fluid Dynamics (CFD) simulations. Transient analysis with transient rotor stator frame change model and shear stress transport turbulence model are utilized in ANSYS CFX. An experimentally validated numerical model with full scale tidal turbine with a blockage ratio of 14.27% and Tip Speed Ratio (TSR) 4.87 is used to simulate the effect of different tower diameters on performance and wake. The effect of different tower diameters is quantified in terms of coefficient of performance (C_P). Coefficient of performance for a 3.5 m tower diameter is 0.472 which is followed by 3, 2.5 and 2 m with coefficients of performance of 0.476, 0.478 and 0.476 respectively. Similarly, the coefficient of thrust (C_T) on the rotor for 3.5 m tower diameter is 0.902, for 3 m diameter 0.906 and for 2.5 and 2 m diameters are 0.908 and 0.906 respectively.