1. An image processing apparatus for processing a previous image having a plurality of first pixels and a present image having a plurality of second pixels, and outputting an output image, said image processing apparatus comprising: a pixel difference calculator for calculating a difference value between each first pixel and one of the second pixels which position is corresponding to said first pixel so that output a plurality of pixel differences; a counter for counting a number of positive pixel differences and a number of negative pixel differences in the pixel differences of a sampling window; a motion level determining unit for calculating a motion level of a pixel in the sampling window according to the number of the positive pixel differences and the number of the negative pixel differences counted by said counter; a blending value determining unit coupled to said motion level determining unit, and for determining a blending value according to the motion level; and an output unit coupled to said blending value determining unit, and for adding together weights of the present image and the previous image according to the blending value to generate and output an output image.