Morphological transition of the ABC-type triblock copolymer from lameller to tricontinuous Gyroid structure has been observed. The sample used is a symmetric poly(isoprene-block-styrene-block-2-vinylpyridine), whose volume fractions of three components are 0.26, 0.48 and 0.26, and the total molecular weight is 1.0 x 10(5). The solvent cast films were prepared and dried well, followed by annealing at 423 K, the temperature well above the glass transition temperatures of three polymer components. The structural transition was investigated by varying the annealing time from 1 h up to 7 days with two methods, transmission electron microscopy and small-angle X-ray scattering. It was confirmed that the as-cast film shows lamellar structure, however, the structure started to transform into Gyroid structure after annealing 14 h and it took more than 2 days to accomplish the transition. Furthermore, it was also continued that Gyroid structure is oriented with its (110) normal to the film surface, the length of its repeating unit along with (110), 55 nm, is similar to its original lamellar domain spacing, 58 nm. (C) 2004 Elsevier Ltd. All rights reserved.