A method of controlling a threshold voltage is provided. The method of controlling a threshold voltage includes performing a film-thickness measuring step to measure the thickness of a film layer on a wafer to obtain a film-thickness value. Then, at least one parameter is decided, selected, or generated according to the film-thickness value. Next, an ion implantation process is performed on the wafer, wherein the ion implantation process is executed according to the parameter to form a threshold voltage adjustment region in the wafer below the film layer.