This report describes a quantification method for luminance non-uniformity of a large LED backlight. In experiments described herein, participants subjectively evaluated artificial indistinct Mura images that simulated luminance non-uniformity of an LED backlight. We measured the luminance distribution of the Mura images. Then, the measured luminance distribution was converted into S-CIELAB, in which anisotropic properties of the spatial frequency response of human vision were considered. Subsequently, some indexes for the quantification model were extracted. We conducted multiple regression analyses using the subjective evaluation value and the index values obtained from measured luminance of Mura image. We proposed a quantification model consisting of four indexes: high and low luminance area, number of Mura edges, sum of Mura edge areas, and maximum luminance difference.