A method of and apparatus for controlling a transfer voltage based on a paper specific resistance in a laser beam printer is provided. In the transfer voltage controlling method, the resistance of the feed rollers is detected as a first resistance before a paper sheet passes between the feed rollers. The resistance of the paper sheet and the feed rollers is detected as a second resistance when the paper sheet passes between the feed rollers. The specific resistance of the paper sheet is obtained as a third resistance by determining the difference between the first resistance and the second resistance. The resistance between a transfer roller and a photosensitive drum is detected as a fourth resistance before the paper sheet passes through the transfer roller. A composite resistance is detected as a fifth resistance by adding the third resistance to the fourth resistance, and the paper sheet is printed by applying a transfer voltage to the transfer roller based on the fifth resistance.