The kinetics and mechanism of morphological transition from nonequilibrium lamella to cylinder microdomain in a polystyrene-block-poly(ethylene-co-but-1-ene)-block-polystyrene (SEBS) triblock copolymer were studied by using time-resolved synchrotron small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and rheology. The sample cast from toluene solution formed a nonequilibrium morphology of the alternating lamellae (LAM) of polystyrene (PS) and poly(ethylene-cobut-1-ene) (PEB) blocks because toluene is a good solvent for PS chains but a poor solvent for PEB chains. LAM of PS and PEB blocks was transformed into the hexagonally close packing (HEX) of PS cylinders in PEB matrix when the as-cast sample was annealed above 140 degreesC, which is above the glass transition temperature of the PS block. From the time-resolved SAXS and TEM, the coexistence of the LAM and HEX microdomains was clearly confirmed during the entire process of the order-to-order transition (OOT) from LAM to HEX, and the lamellar fraction in the coexisting phase decreased with increase of the annealing time. It was also found that the storage modulus during the transition linearly decreased with the lamellar fraction. The temporal change in the LAM fraction in the coexisting phase was fitted by the Avrami-type exponential decay function. The mechanism of the OOT was discussed on the basis of the evaluated Avrami exponent (n). We found that the value of n was similar to1 at higher annealing temperatures (above 160 degreesC), whereas it was similar to0.5 at lower annealing temperatures. Thus, it is concluded that at higher temperatures the nucleation and growth of the HEX microdomains from LAM phase result from both interlayer correlation of the modulation and in-layer modulation of LAM layers (2-dimensional growth). On the other hand, at lower temperatures, one-dimensional growth from the in-layer modulation induced the nucleation and growth of HEX microdomains.