The relationship between structure development and mechanical properties of NBR/silica compounds was studied as a function of acrylonitrile content of NBR and of mixing procedures. FT-IR spectra and transmission electron microscopy (TEM) observations revealed that hydrogen bonding between nitrile groups in NBR and silanol groups on the silica surface enhanced the dispersion of silica particles in the rubber matrix. At a given acrylonitrile content of NBR, the amount of filler-gel content decreased and simultaneously the amount of rubber-gel increased with the increase of mechanical mixing time. The dynamic storage modulus for the compounds was found to be closely related to the amount of filler-gel in the compounds.