An image processing method and its associated image processing circuit for processing an image based on a sequential couleur avec memoire (SECAM) system are provided. The image includes a first pixel, a second pixel and a third pixel, which are successively arranged in a same vertical line and are respectively corresponding to a first image signal and a second image signal and a third image signal. The image processing method includes steps of calculating a chroma signal via a vertical filtering process according to the first image signal, the second image signal and the third image signal; calculating a chroma angular frequency via a frequency modulation process according to the chroma signal; and generating a chromaticity according to the chroma angular frequency.