In this investigation the effects of a rosin-type nucleating agent, which was prepared from cocrystallizing of dehydroabietic acid and Na-dehydroabietate, on polypropylene (PP) crystallization were studied. The results of differential scanning calorimetry and X-ray diffraction proved that a cocrystal of dehydroabietic acid and Na-dehydroabietate was formed. The lower melting point of the cocrystal caused it to be uniformly dispersed in PP. When cocrystals were added as nucleating agent, the mechanical properties, heat distortion temperature, and crystallization temperatures of PP were obviously improved, and the size of spherulites was also decreased. This proved that the cocrystals of dehydroabietic acid and Na-dehydroabietate could act as an effective nucleating agent for PP.