High impact polystyrene was synthesized using two series of styrene/butadiene (SB) tapered block copolymers with a polystyrene (PS)/polybutadiene (1313) composition of 30/70 and 10/90 wt%. During the synthesis, concentration of initiator, SB and transfer agent were varied. From dynamic mechanical analysis, the corresponding a relaxation of the rubber phase was detected at low temperature (near -100 degrees C) and that of the glassy PS phase at high temperature (near 100 degrees C). Also, another relaxation at temperature near 40 degrees C was identified, which was associated to the beta relaxation of the glassy PS phase. The variations found in the alpha relaxation of the rubber phase, were attributed to changes in the morphological structure as a consequence of variation in initiator, SB or transfer agent concentrations and in SB composition. beta relaxation showed a strong dependency with the interfacial state between the rubber and the glassy phase, where an increase in the amount of graft PS at the interface, which promotes the interfacial adhesion between phases, causes an increase in the magnitude of beta relaxation of the PS phase. The results were attributed to variations in the interfacial area as a result of the change in the particle size and to the contribution of molecular chains of each phase in participating in the relaxation process.