Abstract: The structure and dynamic mechanical properties of unpressed and pressed composites consisting of carboxylated nit rile rubber (XNBR) and hindered phenol compound 2，2'-methylenebis-(6-tertbutyl-4-methyl-phenol) (AO-2246) were investigated respectively. The results show that the dispersion state of AO-2246 in the XNBR matrix has an important influence on the dynamic mechanical properties of the composites. In unpressed AO-2246/XNBR composites，most AO-2246 molecules exist in the form of crystalline particles，which leads to alower tanδ value than that of the pure XNBR matrix. On the contrary，in the pressed composites，all AO-2246 molecules are hybridized with XNBR during the hot-pressing and quenching process，so the tanδ value is increased remarkably and the damping property is improved significantly. DSC measurement and SEM observation show that in the AO-2246/XNBR hybrids with a larger AO-2246 mass fraction than 40%，AO-2246 molecules can be divided into three kinds：hybrid，microseparated，and crystal，which affect the dynamic mechanical properties of the composites together. The peak value of the tanδ peak of AO-2246/XNBR (50 wt%) hybrid reaches 3.5，meanwhile，its peak position also shifts to room temperature，demonst rating that AO-2246/XNBR composite is a promising high-performance damping material.