In this paper, the vibration damping properties were measured by cantilever method with steel beams as substrate, gradient PU/VER (BMA) IPN as coatings and when used, polysulfide rubber modified epoxy resin without fillers and with common inorganic fillers and whisker crystals as constrained layer. The effects of the thickness ratio of damping layer and steel beam and the sequence of gradient coating on loss factor (eta) of extensional damping structure were studied. The effects of the thickness ratio and the time interval of coating (the time difference between coating a layer and another layer) between constrained layer and damping layer on damping properties of constrained damping structure were detected. Modulus of constrained layer was further increased by adding common fillers and inorganic whisker crystals in order to increase eta of overall structure. The results show that damping properties of the extensional damping structure with the thickness ratio of 2:1 and the sequence of 70:30-60:40-50:50 (on the steel beam, the first layer is IPN with the component ratio of 70:30, the second layer is 60:40 IPN and the third layer is 50:50 IPN), are better compared with the others. When the thickness of constrained layer and damping layer is respectively 1 mm and the time interval of coating is 3 h, the eta of optimized constrained damping structure with 10% aluminum borate (Al18B4O33) whisker crystal in the constrained layer at 2nd mode are higher than 0.14 from -20 to 55 degrees C and its eta at 2nd mode is 0.32 at -20 degrees C. The binding condition between constrained layer and damping layer was observed by SEM. (c) 2005 Elsevier B.V. All rights reserved.