Method for converting gradation data of video signal

A method for converting gray level data of a video signal is provided to divide each frame overlappingly output to a display unit into a plurality of sub frames, and process gray level conversion by making a sum of gray level data of the respective sub frames equal to that of the frames, thereby reducing a data processing load of a display driving unit and reducing a size of the display driving unit. A method for converting gray level data of a video signal includes the steps of dividing gray level data of frames into a plurality of sub frames, sequentially setting data values of upper bits of the frames in bits of upper frames except lowermost bits of the respective sub frames, and setting data values corresponding to lower bits of the frames which are not set in upper bits of sub frames in lowermost bits of the other sub frames except one sub frame one by one in order to make a sum of gray level data of the respective sub frames equal to that of the frames. When the frame is formed of N bit gray level data and divided into K sub frames, L is a natural number larger than 1, K=2^L, and a maximum bit number of the sub frames is N-(L-2).