An image signal processing apparatus and a method thereof are disclosed. The image signal processing apparatus includes a random producing unit which produces seed values to a plurality of frames input for a predetermined period by using a linear feedback shift register (LFSR), and a dithering processing unit which carries out a dithering to input image signals by using the seed values produced by the random producing unit. With this construction, the apparatus can carry out the dithering without using a frame buffer, thereby allowing a high-definition image to be realized.