An image processing method is provided. An image signal is received and correspondingly converted to an original luminance value, an original first chroma value and an original second chroma value. The original luminance value is adjusted to a modulated luminance value. The modulated luminance value is larger than the original luminance value as the original luminance value is larger than a threshold. The modulated luminance value is less than the original luminance value as the original luminance value is less than the threshold. The original first and second chroma values are linearly adjusted to a modulated first chroma value and a modulated second chroma value according to an original vector between the original first and second chroma values and a white point in a color gamut. The modulated luminance value, the modulated first chroma value and the modulated second chroma value are reconverted to a modulated image signal.