We studied the crystallization kinetics in a diblock copolymer system exhibiting different mesophase structures in the melt. A symmetric poly(ethylene oxide)-block-poly(1,4-butadiene) (PEO-b-PB) was blended with a low molecular weight PB homopolymer to yield the block copolymer blends containing lamellar, cylindrical, and spherical PEO microdomains. The crystallization kinetics of PEO blocks in these nanoscaled microdomains was then studied by monitoring the development of crystallinity in the course of isothermal crystallization. In the lamellar melt, crystallization could occur at the normal undercooling, and its kinetics closely followed the classical Avrami model found in the spherulitic crystallization of homopolymers. Crystallinity developments in the cylindrical and spherical morphology obeyed a simple exponential function prescribed by the first-order kinetics. This first-order kinetic behavior along with the exceedingly large undercooling verified the homogeneous nucleation controlled kinetics in these two types of mesophases. Crystallization in the lamellar melt transformed the melt structure into a highly interconnected lamellar morphology due to the ability of the crystal growth fronts to repeatedly thrust into the microdomains yet to be crystallized. For the crystallization condition chosen (i.e., cooling at -5 degreesC/min from the melt), the melt structures associated with the cylindrical and spherical morphology were not totally disrupted and transformed into one-dimensionally stacked lamellae upon crystallization. The melt mesophases were not fully preserved either, suggesting that some intermediate structures may have been formed through the crystallization.