Nitrile rubber (NBR) compounds containing various loadings of fly ash (FA) were prepared, and their viscoelastic properties and reinforcement mechanism were investigated and compared with those of natural rubber (NR) compounds reported previously. The results obtained exhibit an increase in storage modulus (G') with increasing FA particularly at high FA loading. By contrast, the broadness of linear viscoelastic (LVE) region is found to decrease. With the use of Guth-Cold equation, the positive deviation of experimental values of relative modulus outward the theoretical values is observed at low strain of deformation. However, at high deformation strain, the negative deviation is found. The results imply that the presence of pseudo-network (as formed via FA-FA and FA-NBR interactions) and the ball bearing effect provided by FA having spherical shape are responsible for the reinforcement in FA filled NBR compounds. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3497-3502, 2010