An apparatus for auto white balance includes a histogram generation unit writing respective histograms of Cb values and Cr values with regard to pixels included in an input image frame, a single-tone determination unit comparing respective maximum values of the histograms of Cb values and Cr values with a first threshold value, and determining the input image frame as a single-tone image if at least one of the maximum values is higher than the first threshold value, and an auto white balance gain computation unit computing and an auto white balance gain by using color information of an input image frame and outputting the computed auto white balance gain, wherein if the input image frame is determined to be a single-tone image, the auto white balance gain computation unit outputs an auto white balance gain computed with respect to a previous input image frame.