Brightness correction apparatus and method for a plasma display, where the non-linear relationship between the gray scale and the brightness of the plasma display is considered. The brightness error is measured to build up a brightness error table. When the gray scale data of the currently displaying pixel is received, the brightness error diffusion method is applied. The weighted display brightness error of the neighboring pixel is incorporated for calculation to obtain an optimal display result. When the modified output gray scale data is derived by calculation, the brightness error table is looked up, and the display brightness error of the currently displaying pixel is thus saved to provide modification calculation for other pixels.