1. A method of motion detection using an adaptive threshold, adaptive for determining whether a plurality of target pixels are moving pixels or not, the method comprising: giving a first threshold for determining whether the target pixels in a current scan line are moving pixels, wherein the first threshold has an upper limit and a lower limit; calculating a number of moving pixels in a previous scan line of the current scan line; determining whether the number of moving pixels is larger than a second threshold; if the number of moving pixels is larger than the second threshold, adjusting the first threshold by a decrement; and determining whether the target pixels in the current scan line are moving pixels by using the adjusted first threshold, wherein after the step of giving the first threshold, the method further comprises: adjusting the first threshold by an increment when the motion detection of every L scan lines is completed, wherein L is a positive integer.