1. A method of measuring image-sticking phenomenon of a display, comprising: providing a display having N gray levels; displaying an image-stick test frame on the display, wherein the image-stick test frame comprises at least a first pattern having a low gray level and at least a second pattern having a high gray level; displaying the image-stick test frame for a predetermined time period so that an image-stick region and a non-image-stick region are formed on the display, wherein the image-stick region is located at a region where one of the first and the second patterns is positioned, and the non-image-stick region is located at a region where another one of the first and the second patterns is positioned; displaying a measuring frame on the display, wherein the non-image-stick region in the measuring frame has a standard gray level M, and sequentially displaying a plurality of middle gray levels on the image-stick region in the measuring frame, wherein the middle gray levels are between M and M+1 or between M−1 and M; and judging which one of the middle gray levels make a boundary between the non-image-stick region and the image-stick region in the measuring frame lightest, and then converting the middle gray level into an image-sticking level.