Experimental and simulation studies have been conducted about the effect of apex cone height on particle separation performance of a cyclone separator. It is found that the effect of an apex cone is to decrease the cut size and to increase the collection efficiency. The cut size indicates the minimum value for the specific height of the apex cone. The optimum apex cone height changes to the lower position as the cyclone inlet velocity increases. When the apex cone is set to the high position, the resistance of an incoming particle entering to the dust box increases. The low apex cone position induces a strong upward fluid flow component and increases the number of escaping particles from the dust box. The magnitude of fluid velocity in the upper part of the dust box decreases under the optimum apex cone height condition. The experimental data agreed with the numerical simulation based on the direct flow calculation method.