The present invention provides an image processing method and a device thereof for processing the image comprising a plurality of pixels, wherein each image has an alpha value and image data. The method of the invention comprises the following steps: selecting a first pixel and a second pixel which are adjacent; selecting the alpha values of the first pixel and the second pixel; detecting the image data of the first pixel and the image data of the second pixel; determining a plurality of interpolated pixels according to the first pixel and the second pixel; generating a weighted value corresponding with each pixel in the plurality of interpolated pixels, wherein the plurality of weighted values are different from each other; and adjusting the image data of the interpolated pixels to be the same as the image data of the first pixel or the image data of the second pixel if one selected from the first pixel and the second pixel has a zero alpha value while the other alpha value is not 0. The image processing method and the device of the invention can determine the data image of the interpolated pixels based on the alpha value and the weighted value. Therefore, the rooming image of the invention has better definition.