Aerodynamic performance of a wind turbine can be significantly affected by wind turbulence in urban and built-up area, the atmospheric wind becomes highly turbulent and the effect of turbulence on the aerodynamic performance of a vertical axis wind turbine was studied via wind tunnel testing under controlled levels of wind turbulence. The goal of this paper is to study the effect of turbulence on a H-type vertical axis wind turbine using NACA0021 shaped blades under variable wind speeds and for experimental testing demonstrate using Subsonic, open-type wind tunnel which has an over whole dimension of wind tunnel 2890mm x 860 mm x 1670 mm, and a testing section of 292 x 292 mm in the test area and the wind tunnel only suitable for relatively low air velocities (0 < Mach< 0.2) for measuring RPM, lift and drag force on the turbine blades . For the numerical approach, ANSYS Fluent simulations were used for analyzing aerodynamic performance by utilizing sliding mesh model techniques, the models were designed using the CAD software SolidWorks. k- ε. Turbulence model employed for the analysis and simulation is set to run at various tip speed ratios. The turbulence was generated inside the wind tunnel using with a wooden grid. The results of simulation show that produced power increases with the wind speed. Turbulence levels had a small but noticeable impact on overall power output. Low turbulence levels resulted in increased power output at wind speeds. High levels of turbulence do have a significant effect on turbine performance, causing a drop of power. The average coefficient of power decrement is 29.8%,45% and 77.6% large,medium and small grids respectively relative to free turbulence flow. An appropriate validation is made by comparing CFD results with the experimental results and a good agreement with less than 6 percent of discrepancy in average is observed.