A gain-determining method for grayscale white balance of a display apparatus comprising a panel is provided. The method first displays a white, a red, a green, and a blue images on the panel respectively and measures the chromatic values and luminance of the images. Next, the chromatic values and luminance of a target white image is inputted. Following, according to the measured chromatic values and luminance of the white, the red, the green and the blue images, a first set of color mixture ratios is calculated. Then, according to the chromatic values and luminance of the target white image and the chromatic values and luminance of the red, the green and the blue images, a second set of color mixture ratios is calculated. Finally, according to the first set and the second set of color mixture ratios, a red gain, a green gain and a blue gain are calculated.