An apparatus to control color registration and image density and a method of calculating a color registration error. The apparatus includes registration marks formed on a transfer belt such that each of the registration marks includes a scan direction component and a slanting direction component at an angle with respect to both the scan direction and the cross-scan direction. The apparatus further includes image density marks formed on the transfer belt, having predetermined image densities, and a registration and image density sensor provided above the registration marks and the image density marks to radiate beams onto the registration marks and the image density marks. The sensor receives beams reflected from the registration marks and the image density marks to produce detection signals, and obtains registration information and image density information from the detection signals. Accordingly, X-offset, Y-offset, printing width error, and skew can be simultaneously compensated for using a single apparatus.