Rheological properties were examined for polybutadiene-polystyrene (PB-PS) diblock Copolymer micelles that had soft PB cores and were dispersed randomly in a matrix of low-M homopolystyrene (hS) plasticized with dibutyl phthalate. In the linear viscoelastic regime, the micellar dispersions exhibited fast and slow relaxation processes attributed to the orientational relaxation of individual Corona blocks and the relaxation of the Brownian stress (occurring through the micelle diffusion), respectively. Under large step strains, the fast and slow processes exhibited nonlinear damping in their relaxation moduli, and the damping was much stronger for the slow process than for the fast process. Under fast flow, two-step thinning of the viscosity corresponding to this damping behavior was observed. All these linear and nonlinear rheological features were quantitatively similar to those of PS-polyisoprene (PI) copolymer micelles having glassy PS cores. Analysis based on the emulsion model suggested that the interfacial relaxation of the soft PB cores of the PB-PS micelles occurred rapidly (almost together with the Corona relaxation) because the PB cores had only small diameters (< 40 nm). Thus, in long time scales after this interfacial relaxation, the PB core effectively behaved as a rigid core (keeping its spherical shape due to the segregation power/surface tension against the hS matrix), thereby exhibiting the above similarities with the PS-PI micelles having glassy PS cores.