The invariant moments of an image, which exhibit three inherent properties, i. e. the invariance under image shifting, rotating and scaling, are described. And a hybrid optical digital processor consisting of a mask, two lenses, a CCD detector and a microcomputer, is set up for extracting the invariant moments of the image. The experimental results show that the measured invariant moment values of six English printing letters are indeed unchanged under letters shifting and rotating. In consequence the processor can be applied to pattern recognition for image shifting and rotating.