A color image forming apparatus and a control method thereof to correct an error between scanning lines due to an alignment error between the laser scanning units corresponding with each color of the image forming apparatus on the basis of a difference of detection times between termination signals for each color when the scanning units output a horizontal synchronization signal and a termination signal to scan a line. A difference of termination times between termination detecting sensors may be used to variably set video clocks for each color, thus reducing the number of components required and reducing the signal processing load. A single sensor may be used to detect the synchronization and termination signals, thus further reducing manufacturing costs.