Abstract In this work, the combined effects of polydispersity and viscosity on microstructure that evolves with domain coarsening and crystallization are investigated in the blends of amorphous solution polymerized styrene-butadiene rubber (SSBR) and crystalline trans-1,4-polyisoprene (TPI). We find that the bicontinuous patterns in SSBR/TPI undergo a complicated coarsening process and evolve to microstructures where high density SSBR drops are trapped in TPI-rich domains after the secondary phase separation, which is the result of the hydrodynamic difference between TPI components with different molecular weights and the viscosity difference between SSBR and TPI. Domain interface assists crystallization by acting as the substrate for the static heterogeneous nucleation. Although TPI spherulites preferentially grow in TPI-rich domains with SSBR drops in-situ included in crystals, they still could cross over SSBR-rich phases. Owing to the polydispersity of TPI, phase separation driving low-molecular-weight TPI assembling in TPI-rich domains enables the predominant formation of polymorphic α-TPI by lowering the crystallization energy barrier.