An image signal processor and a method for processing an image signal thereof are disclosed. When the image signal processor executes an automatic chroma gain control (ACC), the image signal processor adjusts a variable rate of ACC gain according to a size of an input color signal to reduce a time for processing the ACC. Even if a difference between the size of the input color signal and the size of a reference color signal is large, the ACC is rapidly processed. As a result, transient phenomenon disappears from a screen.