The effect of tackifier on the adhesive properties of a model pressure-sensitive adhesive tape was investigated. For this purpose, a model system consisting of poly(styrene-isoprene-styrene) triblock copolymer as the base polymer and a typical aliphatic petroleum resin as the tackifier was prepared. The tackifier content ranged from 10 to 60 wt%. The tackifier used has a good compatibility with polyisoprene, whereas it has a poor compatibility with polystyrene. The 180 degrees peel adhesion was measured. The peel adhesion increased with the tackifier content, while the degree of increase became more significant above 40 wt%. The pressure sensitivity appeared obviously and the maximum peel adhesion was obtained without heating above 40 wt%. The phase structure was determined using pulse H-1-NMR, transmission electron microscopy and dynamic mechanical analysis. A phase structure in which spherical polystyrene domains with a mean size of about 20 nm were dispersed in the polyisoprene continuous phase was observed. It was found that the tackifier-rich phase of the order of nanometers in size was formed in the polyisoprene matrix and the concentration increased with the tackifier content. The tackifier-rich phase seemed to develop the cohesive strength and, thus, it increased the peel adhesion.