Natural rubber (NR) was blended in various ratios with 29 kinds of tackifier resins, which were prepared from rosin, terpenes, and petroleum. Miscibilities of all the blend systems were illustrated as phase diagrams. From these blend systems, we selected 7 systems having typical phase diagrams [completely miscible, completely immiscible, and lower critical solution temperature (LCST) types] and carried out measurements of peel strength. Peel strength was measured at the angle of 180 degrees at 20 degrees C over the wide range of pulling rates. In the case of pressure-sensitive adhesives (PSAs), which showed phase diagrams of the completely miscible or LCST type, the peak positions in the pulling rate-peel strength curves shifted to the lower velocity as the tackifier content increased. On the contrary, completely immiscible PSAs had a smaller peel strength than miscible ones and did not give manifest shift of peaks. In most of the adhesives, the fracture mode changed from cohesive failure to interfacial failure (between adhesive and adherend), slip-stick failure, and glassy failure (between the tape and adhesive) as the pulling rate increased.