Adhesion mechanism of a polyacrylic block copolymer/tackifier blend as a model pressure-sensitive adhesive was investigated. Special rosin ester resins with different weight average molecular weights of 650, 710, 890 and 2,160 were used as the tackifier and blended with a polyacrylic block copolymer consisting of poly (methyl methacrylate) and poly (n-butyl acrylate) blocks at tackifier content levels of 10, 30 and 50 wt%. From a transmission electron microscopic observation, the number of formed agglomerates of the tackifier with sizes on the order of several tens of nanometers increased with increasing tackifier content and molecular weight of the tackifier in the range from 650 to 890. For the tackifier with a molecular weight of 2,160, micrometer-sized agglomerates were observed. The glass transition temperature of adhesive measured by a dynamic mechanical analysis increased dependent on the number of formed nanometer sized agglomerates. Tack was measured using a rolling cylinder tack tester over wide temperature and rolling rate ranges, and master curves were prepared in accordance with the time-temperature superposition law. Tack increased dependent on the number of formed nanometer sized agglomerates. It was found that the adhesion properties are strongly affected by the nanometer sized agglomerates of tackifier. That is, the structure of a pressure-sensitive adhesive with tackifier is like nanocomposite.