An apparatus and method for video data processing in digital video decoding, such as digital video disk (DVD) decoding, which produces new video data according to main picture data, sub-picture unit (SPU) data, and an aspect ratio. The apparatus includes a sub-picture-unit-decoding and color-determining device, a mixer, a buffer, and an interpolator. The sub-picture-unit-decoding and color-determining device is used for performing sub-picture decoding and color determination on the SPU data, and producing decoded SPU data. The mixer, coupled to the SPU-decoding and color-determining device, is employed to mix the main picture data and the decoded SPU data, and outputting combined video data. The buffer, coupled to the mixer, for storing the combined video data outputted from the mixer. The interpolator is used to receive the combined video data from the mixer and to read previously combined video data from the buffer, and to generate the new video data by performing an interpolation operation according to the aspect ratio, the combined video data, and the previously combined video data.