A method of motion detection using an adaptive threshold, adaptive for determining whether a plurality of target pixels in a current scan line are moving pixels or not, is provided. First, a first threshold is given for determining whether the target pixels in the current scan line are moving pixels. Then, a number of moving pixels in a previous scan line of the current scan line is calculated and determined whether larger than a second threshold. If the number of moving pixels is larger than the second threshold, the first threshold is adjusted by a decrement, and then whether the target pixels in the current scan line are moving pixels is determined by using the adjusted first threshold.