During recent years, considerable attempts have been made to apply the natural ventilation due to the increase of the human concern about environmental pollution and the need for efficient energy sources. Ventilation with the use of renewable energies and without a need for mechanical system is defined as the natural ventilation such as wind-catcher and solar chimney. Wind-catchers as environmental friendly and sustainable energy systems are traditionally used in the Middle East countries especially in central region of Iran for providing fresh air of buildings. Wind-catchers also reduce environmental pollution and cooling costs; and they create a favorable and healthy environment for residents, compare with mechanical ventilation systems such as air conditioning and chiller. Wind-catchers are able to catch fresh air with high velocity and less dust at high altitude and induce it from the windward channels into the building. In this study, natural ventilation performance of a two-sided wind-catcher is investigated for different wind angles (0°-90°) and wind speeds (10-20 m/sec) by experimental wind tunnel and smoke flow visualization. To achieve this purpose a 1:50 scale model of a real wind-catcher in the city of Yazd (central region of Iran) is employed. The ventilated flow rate and the pressure coefficients around all internal surfaces of a two-sided wind-catcher are measured in order to find the influences of the wind direction and wind speed on the ventilation capacity. It is found that the averaged pressure coefficients decrease by increasing the wind angle and their maximums occur at a zero wind angle. There is a wind angle where the pressure coefficient of the windward opening is equal to that for surrounding. This angle is about ?=55° and is denoted as the windward transition angle where no airflow passes through the windward opening and only air flows between the window and the leeward opening. It is also found that no airflow passes through the window at the wind angle of 39° (window transition angle). Flow rate through the windward and leeward channels of the wind-catcher for different wind angles and wind speeds are calculated based on the velocity measurement by hot-wire anemometer. The results show that the capacity of wind-catcher increases with increasing the external wind speed. Wind tunnel experiments and smoke flow visualization show the separated flow, wake region, Kelvin–Helmholtz (KH) vortices on the separating shear layer over the wind-catcher and the formation of two opposite vortices inside the wind-catcher. These two vortices are symmetric and small in size at ?=0°. At higher wind angle, they become asymmetric and larger in size. The results also show that the performance of the wind-catcher in the high-density urban areas is dependent on the upstream structure height and its distance from the wind-catcher. The results show that using the short upstream object from the wind-catcher increases the airflow to the building. Employing the upstream objects with height greater than half the height of the wind-catcher causes the performance of the wind-catcher is reduced. Keywords: Natural Ventilation, Two-Sided Wind-Catcher, Wind Tunnel, Smoke Tunnel, Smoke Flow Visualization.