Tack behaviours of p-t-octylphenol formaldehyde resin with rubbers, such as cis-l,4-polyisoprene and cis-1,4-polybutadiene, were studied using molecular mechanics and dynamics. The structures of p-t-octylphenol formaldehyde resin were found to be that hydroxyl groups cluster in the centre of the molecule by intramolecular hydrogen bondings and the t-octyl groups are extended out. The tack of p-t-octylphenol formaldehyde resin with rubber is formed by intermolecular non-bond interactions between the t-octyl groups of the resin and rubber chains. The interaction energies between one p-t-octylphenol formaldehyde resin molecule and two rubber molecules were calculated to investigate the effect of the molecular size of the resin on the tack strength. The interaction energies for the dimer and the trimer of p-t-octylphenol formaldehyde resin are greater than - 20 kcal/mol while those for the tetramer-decamer are less than - 40 kcal/mol.