Abstract: In surface inspection of medium and heavy plates, defects recognition based on texture analysis suffers from mutative illuminations and scales. An illumination invariant texture analysis method named structure spectrum was proposed and applied to recognition of surface defects on medium and heavy plates. Compared with other textural features such as gray level co-occurrence matrix, Laws texture energy, and Fourier power spectrum, higher classification rates were made by structure spectrum for classification of pits, scars and inclusions.