In this paper an image authentication technique that embeds a binary watermark into a host color image is proposed. In this scheme the colour image is first transformed from the RGB to the YCbCr color space. The watermark is embedded into the Y channel of the host image by selectively modifying the very low frequency parts of the DCT transformation. It is shown that the proposed technique can resist classical attacks such as JPEG Compression, low pass filtering, median filtering, cropping, and geometrical scaling attack. Moreover, the recovery method is blind and doesn't need the original host image for extraction. © 2009 IEEE.