The agglomeration of spherulites during isothermal crystallization of olefin multiblock copolymers and the role of mesophase separation on the agglomeration behavior were investigated by a new rheological method, which is based on the analogy between crystal morphology and particle suspension in polymeric matrix. The new suspension-based rheological method solved the problem of frequency dependency, which was encountered in the traditional rheological study in determination of the fraction of transformation. It was achieved by decomposing the time-resolved dynamic moduli during crystallization into the hydrodynamic part and the agglomerates' part using a two-step shifting procedure. The relative crystallinity determined from the product of two shifting factors, the strain rate amplification factor and the stress amplification factor, was consistent with the DSC measurements. Moreover, the dependence of the agglomerates' contribution to the storage modulus of the crystal (G'(Agg)) on its volume fraction (phi(rheo)) was found to be independent of the crystallization temperature, resulting in a master curve G'(Agg) vs phi(rheo) that could be used as a unique parameter to characterize the agglomeration of spherulites. For olefin block copolymers with similar hard-block content (or crystallinity), it was found that mesophase separation not only delayed the agglomeration of spherulites but also changed its packing behavior. Comparisons with polymer nanocomposites further illustrated the differences in the spatial distribution and agglomeration of "fillers" in polymer nanocomposites, homogeneous semicrystalline polymers, and heterogeneous semicrystalline polymers.