The crystallization kinetics of the polyethylene block within polyethylene-b-polystyrene (PE-b-PS) and polyethylene-b-poly(ethylene-alt-propylene) (PE-b-PEP) diblock copolymers has been investigated in a wide composition range by differential scanning calorimetry (DSC) and compared to an equivalent homopolymer. The morphology of the copolymers in the melt and its changes after crystallization were explored by small-angle X-ray scattering (SAXS) experiments. SAXS experiments demonstrated that PE-b-PS formed microdomain ordered structures in the melt. On the basis of the lamellar morphology that was developed by PE-b-PEP (demonstrated by transmission electron microscopy, TEM) regardless of composition, it was deduced that these diblocks crystallize from either a homogeneous or a weakly segregated melt. Also, the effects of the neighboring block on thermal fractionation of the PE block were obtained by successive self-nucleation and annealing (SSA). Classical kinetics theories of polymer nucleation and crystallization were applied to DSC overall isothermal crystallization data. The energetic parameters obtained allow us to quantitatively estimate the increase in the energy barrier for crystallization of the PE block, caused by the covalently bonded PEP and PS block as compared to homo-PE. Also, the Lauritzen and Hoffman theory was applied to DSC isothermal crystallization of previously self-nucleated PE (where crystal growth is the dominant factor), and the contributions of nucleation and growth were estimated. Using this novel approach, we demonstrate that the nucleation process is the rate-determining step in the crystallization behavior of highly confined PE-b-PS diblock copolymers, results that were corroborated by the low Avrami index values found in these diblock copolymers. Similar trends were obtained by applying polymer nucleation theories. In the case of PE-b-PEP diblock copolymers, the energy barrier associated with nucleation and growth increases as the PEP content within the copolymers increases. The PEP block acts as a diluent for the PE block crystallization in view of the miscibility between the blocks.