A circuit for automatically detecting image flicker includes an image sensor, an average unit, a difference generation unit, a transform unit, and a flicker detection unit. The image sensor captures an image frame. The average unit generates a luminance average value according to a luminance sum of predetermined pixels of each pixel row of an image frame. The difference generation unit generates a corresponding difference according to a luminance average value of each pixel row of two consecutive image frames. The transform unit performs a discrete Fourier transform on a difference corresponding to a corresponding pixel row of the two consecutive image frames to generate a transform result. The flicker detection unit determines whether a flicker is shown in the two consecutive image frames to generate a detection result according to the transform result.